Photochemical Properties of Red-Emitting Tris(cyclometalated) Iridium(III) Complexes Having Basic and Nitro Groups and Application to pH Sensing and Photoinduced Cell Death

Experimental and theoretical studies of pH-responsive iridium(III) complexes of azole and N-heterocyclic carbene ligands

A series of nine luminescent iridium(III) complexes with pH-responsive imidazole and benzimidazole ligands have been prepared and characterized. The first series of complexes were of the form [Ir(ppy)2(N^N)]+ or [Ir(ppy)2(C^N)]+ (where ppy is 2-phenylpyridine and N^N is 2-(2-pyridyl)imidazole or 2-(2-pyridyl)benzimidazole and C^N represents a pyridyl-triazolylidene-based N-heterocyclic carbene ligand). For these complexes, the benzimidazole group was either unsubstituted or substituted with electron-withdrawing (Cl) or electron-donating (Me) groups. The second series of complexes were of the form [Ir(phbim)2(N^N)]+ or [Ir(phbim)2(C^N)]+ (where phbim is 2-phenylbenzimidazole and N^N is either 2,2'-bipyridine or 1,10-phenanthroline and C^N is either a pyridyl-imidazolylidene or pyridyl-triazolylidene N-heterocyclic carbene ligand). UV-visible and photoluminescence pH titration studies showed that changing the protonation state of these complexes results in significant changes in the photoluminescence emission properties. The pKa values of prepared complexes were estimated from the spectroscopic pH titration data and these values show that the nature of the pH-sensitive ligands (either main or ancillary ligands) resulted in a significant capacity to modulate the pKa values for these compounds with values ranging from 5.19-11.22. Theoretical investigations into the nature of the electronic transitions for the different protonation states of compounds were performed and the results were consistent with the experimental results.

Solid-state mechanochemistry for the rapid and efficient synthesis of tris-cyclometalated iridium(iii) complexes

Tris-cyclometalated iridium(iii) complexes have received widespread attention as attractive prospective materials for e.g., organic light-emitting diodes (OLEDs), photoredox catalysts, and bioimaging probes. However, their preparation usually requires prolonged reaction times, significant amounts of high-boiling solvents, multistep synthesis, and inert-gas-line techniques. Unfortunately, these requirements represent major drawbacks from both a production-cost and an environmental perspective. Herein, we show that a two-step mechanochemical protocol using ball milling enables the rapid and efficient synthesis of various tris-cyclometalated iridium(iii) complexes from relatively cheap iridium(iii) chloride hydrate without the use of significant amounts of organic solvent in air. Notably, a direct one-pot procedure is also demonstrated. The present solid-state approach can be expected to inspire the development of cost-effective and timely production methods for these valuable iridium-based complexes, as well as the discovery of new phosphorescent materials, sensors, and catalysts.

Optical Resolution of Carboxylic Acid Derivatives of Homoleptic Cyclometalated Iridium(III) Complexes via Diastereomers Formed with Chiral Auxiliaries

We report on a facile method for the optical resolution of cyclometalated iridium(III) (Ir(III)) complexes via diastereomers formed with chiral auxiliaries. The racemic carboxylic acids of Ir(III) complexes (fac-4 (fac-Ir(ppyCO2H)3 (ppy: 2-phenylpyridine)), fac-6 (fac-Ir(tpyCO2H)3 (tpy: 2-(4'-tolyl)pyridine)), and fac-13 (fac-Ir(mpiqCO2H)3 (mpiq: 1-(4'-methylphenyl)isoquinoline))) were converted into the diastereomers, Δ- and Λ-forms of fac-9 (from fac-6), fac-10 (from fac-4), fac-11 (from fac-6), and fac-14 (from fac-13), respectively, by the condensation with (1R,2R)-1,2-diaminocyclohexane or (1R,2R)-2-aminocyclohexanol. The resulting diastereomers were separated by HPLC (with a nonchiral column) or silica gel column chromatography, and their absolute stereochemistry was determined by X-ray single-crystal structure analysis and CD (circular dichroism) spectra. Spectra of all diastereomers of the Ir(III) complexes are reported. Hydrolysis of the ester moieties of Δ- and Λ-forms of fac-10, fac-11, and fac-14 gave both enantiomers of the corresponding carboxylic acid derivatives in the optically pure forms, Δ-fac and Λ-fac-4, -6, and -13, respectively.

Development of Wireless Power-Transmission-Based Photodynamic Therapy for the Induction of Cell Death in Cancer Cells by Cyclometalated Iridium(III) Complexes

Photodynamic therapy (PDT), a noninvasive method for cancer therapy, involves the generation of reactive oxygen species (ROS) by the photochemical excitation of photosensitizers (PSs) to induce cell death in cancer cells. A variety of PS including porphyrin derivatives and metal complexes such as iridium (Ir) complexes have been reported. In clinical trials, red-near infrared (NIR) light (650-900 nm) is preferred for the excitation of PSs due to its deeper penetration into tissues compared with visible light (400-500 nm). To overcome this limitation, we established a PDT system that uses cyclometalated iridium(III) (Ir(III)) complexes that are excited with blue light in the wireless power transmission (WPT) system. To achieve this, we developed a light-emitting diode (LED) light device equipped with a receiver coil that receives electricity from the transmitter coil through magnetic resonance coupling. The LEDs in the receiving device use blue light (470 nm) to irradiate a given Ir(III) complex and excite triplet oxygen (³O₂) to singlet oxygen (¹O₂) which induces cell death in HeLa S3 cells (human cervical carcinoma cells). The results obtained in this study suggest that WPT-based PDT represents a potentially new method for the treatment of tumors by a non-battery LED, which are otherwise difficult to treat by previous PDT systems.

Post-complexation Functionalization of Cyclometalated Iridium(III) Complexes and Applications to Biomedical and Material Sciences

Cyclometalated iridium(III) (Ir(III)) complexes exhibit excellent photophysical properties that include large Stokes shift, high emission quantum yields, and microsecond-order emission lifetimes, due to low-lying metal-to-ligand charge transfer (spin-forbidden singlet-triplet (³MLCT) transition). As a result, analogs have been applied for research not only in the material sciences, such as the development of organic light-emitting diodes (OLEDs), but also for photocatalysts, bioimaging probes, and anticancer reagents. Although a variety of methods for the synthesis and the applications of functionalized cyclometalated iridium complexes have been reported, functional groups are generally introduced to the ligands prior to the complexation with Ir salts. Therefore, it is difficult to introduce thermally unstable functional groups such as peptides and sugars due to the harsh reaction conditions such as the high temperatures used in the complexation with Ir salts. In this review, the functionalization of Ir complexes after the formation of cyclometalated Ir complexes and their biological and material applications are described. These methods are referred to as "post-complexation functionalization (PCF)." In this review, applications of PCF to the design and synthesis of Ir(III) complexes that exhibit blue -red and white color emissions, luminescence pH probes, luminescent probes of cancer cells, compounds that induce cell death in cancer cells, and luminescent complexes that have long emission lifetimes are summarized.

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.

pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis, Photophysical Properties, Computational Results, and Bioimaging Application

Herein we report four [Ir(N^C)2(L^L)]n+, n = 0,1 complexes (1-4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2'-bipyridine (2), [2,2'-bipyridine]-4,4'-dicarboxylic acid (3), and sodium 4,4',4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1-4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550-670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1-4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Emission Intensity Enhancement for Iridium(III) Complex in Dimethyl Sulfoxide under Photoirradiation

We found emission intensity enhancement for fac-Ir(ppy)₃ (ppy = 2-(2'-phenyl)pyridine) in aerated dimethyl sulfoxide (DMSO) during photoirradiation for the first time. This phenomenon was concluded to be responsible for the consumption of ³O₂ dissolved in DMSO through dimethyl sulfone production by photosensitized reaction using fac-Ir(ppy)₃. A ³O₂ adduct of DMSO molecule was detected by UV absorption measurement and theoretical calculation. We proposed a mechanism for the emission enhancement reaction including ^1,3O₂ molecules and ^1,3O₂-DMSO adducts and validated it through a simulation of emission intensity change using an ordinary differential equation solver.

Pseudo-Tris(heteroleptic) Red Phosphorescent Iridium(III) Complexes Bearing a Dianionic C,N,C',N'-Tetradentate Ligand

1-Phenyl-3-(1-phenyl-1-(pyridin-2-yl)ethyl)isoquinoline (H₂MeL) has been prepared by Pd(N-XantPhos)-catalyzed “deprotonative cross-coupling processes” to synthesize new phosphorescent red iridium(III) emitters (601–732 nm), including the carbonyl derivative Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)Cl(CO) and the acetylacetonate compound Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)(acac). The tetradentate 6e-donor ligand (6tt′) of these complexes is formed by two different bidentate units, namely, an orthometalated 2-phenylisoquinoline and an orthometalated 2-benzylpyridine. The link between the bidentate units reduces the number of possible stereoisomers of the structures [6tt′ + 3b] (3b = bidentate 3e-donor ligand), with respect to a [3b + 3b′ + 3b″] emitter containing three free bidentate units, and it permits a noticeable stereocontrol. Thus, the isomers fac-Ir(κ⁴-cis-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆H₄-py)}, mer-Ir(κ⁴-cis-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆H₃R-py)}, and mer-Ir(κ⁴-trans-C,C′-cis-N,N′-MeL){κ²-C,N-(C₆HR-py)} (R = H, Me) have also been selectively obtained. The new emitters display short lifetimes (0.7–4.6 μs) and quantum yields in a doped poly(methyl methacrylate) film at 5 wt % and 2-methyltetrahydrofuran at room temperature between 0.08 and 0.58. The acetylacetonate complex Ir(κ⁴-cis-C,C′-cis-N,N′-MeL)(acac) has been used as a dopant for a red PhOLED device with an electroluminescence λ_max of 672 nm and an external quantum efficiency of 3.4% at 10 mA/cm².

The proligand 1-phenyl-3-(1-phenyl-1-(pyridine-2-yl)ethyl)isoquinoline is used to generate a new family of neutral phosphorescent red iridium(III) emitters containing a tetradentate ligand, formed by two different bidentate units, and a third bidentate ligand with a good stereocontrol of the resulting [6tt′ + 3b] products. One of the new emitters has been used in the fabrication of an OLED device.

Triplet Excited States Modulated by Push-Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers

A series of novel monocyclometalated [Ir(tpy)(btp)Cl]⁺ complexes (Ir2-Ir5) were synthesized using 2,2':6',2″-terpyridine (tpy) and 2-(2-pyridyl)benzo[b]thiophene (btp) ligands, as well as their derivatives bearing electron-donating tert-butyl (t-Bu) and electron-withdrawing trifluoromethyl (CF₃) groups. Ir2-Ir5 exhibited visible-light absorption stronger than that of the known complex [Ir(tpy)(ppy)Cl]⁺ (Ir1; ppy = 2-phenylpyridine). Spectroscopic and computational studies revealed that two triplet states were involved in the excited-state dynamics. One is a weakly emissive and short-lived ligand to ligand charge-transfer (LLCT) state originating from the charge transfer from the btp to the tpy ligand. The other is a highly emissive and long-lived ligand-centered (LC) state localized on the btp ligand. Interestingly, the excited state dominant with ³LLCT was completely changed to the ³LC state upon the introduction of substituents on both the tpy and btp ligands. For instance, the excited state of the parent complex Ir2 was weakly emissive (Φ = 2%) and short-lived (τ = 110 ns) in CH₂Cl₂; conversely, Ir5, fully furnished with t-Bu and CF₃ groups, displayed intense phosphorescence with a prolonged lifetime (τ = 14 μs). This difference became increasingly prominent when the solvent was changed to aqueous CH₃CN, most probably due to the ³LLCT stabilization. The predominant excited-state nature was switchable between the ³LLCT and ³LC states depending on the substituents employed; this was demonstrated through investigations of Ir3 and Ir4, bearing either the t-Bu or the CF₃ group, where the complexes exhibited properties intermediate between those of Ir2 and Ir5. All of the Ir(III) complexes were tested as photosensitizers in photocatalytic H₂ evolution over a Co molecular catalyst, and Ir5 outperformed the others, including Ir1, due to improvement in the following key properties: visible-light-absorption ability, excited-state lifetime, and reductive power of the one-electron-reduced species against the catalyst.

Photofunctional transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics

This critical review summarises the recent biological applications of transition metal complexes as cellular probes, bioimaging reagents and phototherapeutics.

Bis-cyclometallated Ir(III) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands; influence of substituents and cyclometallating ligands on response to changes in pH

Bis-cyclometallated Ir(iii) complexes containing 2-(1H-pyrazol-3-yl)pyridine ligands have been synthesised. Their absorption is almost unchanged with changes in pH however the emission intensities vary by a factor of up to three and the complexes have emission pKas in the range 8.0 to 10.0. Substituents on the pyrazole have only a minor effect on the emission pKa. Surprisingly the complexes with phenylpyrazole cyclometallated ligands 3aL1-3 showed an intensity decrease with increasing pH (switch off) whilst the corresponding phenylpyridine ones 3cL1-3 showed an increase in emission intensity with increasing pH. Putting electron-withdrawing CF3 substituents on the cyclometallating phenyls reduced the pKa of the complexes to 6.8-7.8, thereby extending the useful pKa range; however, in general it tended to reduce the magnitude of the change in emission intensity. Surprisingly the CF3-substituted complexes also showed a complete reversal in the direction of the intensity change when compared to their respective unsubstituted congeners.

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.

Amphiphilic Cationic Triscyclometalated Iridium(III) Complex-Peptide Hybrids Induce Paraptosis-like Cell Death of Cancer Cells via an Intracellular Ca2+-Dependent Pathway

We report on the design and synthesis of a green-emitting iridium complex-peptide hybrid (IPH) 4, which has an electron-donating hydroxyacetic acid (glycolic acid) moiety between the Ir core and the peptide part. It was found that 4 is selectively cytotoxic against cancer cells, and the dead cells showed a green emission. Mechanistic studies of cell death indicate that 4 induces a paraptosis-like cell death through the increase in mitochondrial Ca²⁺ concentrations via direct Ca²⁺ transfer from ER to mitochondria, the loss of mitochondrial membrane potential (ΔΨ_m), and the vacuolization of cytoplasm and intracellular organelle. Although typical paraptosis and/or autophagy markers were upregulated by 4 through the mitogen-activated protein kinase (MAPK) signaling pathway, as confirmed by Western blot analysis, autophagy is not the main pathway in 4-induced cell death. The degradation of actin, which consists of a cytoskeleton, is also induced by high concentrations of Ca²⁺, as evidenced by costaining experiments using a specific probe. These results will be presented and discussed.

Redox-Active Bis-Cyclometalated Iridium(III) Complex as a DNA Photo-Cleaving Agent

The development of new photoactive metal complexes that can trigger oxidative damages to the genetic material is of great interest. In the present paper, we describe the detailed study of a highly photo-oxidant iridium(III) complex that triggers photoinduced electron transfer (PET) with purine DNA bases. The PET has been studied by luminescence and laser flash photolysis experiments. From plasmid DNA agarose gel electrophoresis experiments, we demonstrated the high ability of the iridium complex to induce strand breaks upon light irradiation. Reactive oxygen species (ROS)-specific scavengers and stabilizers were employed to identify that the photocleavage process, the results of which infer singlet oxygen and hydrogen peroxide as the predominant species. To the best of our knowledge, the present work represents one of the few study for highly photo-oxidant bis-cyclometalated iridium(III) complex toward DNA.

Photooxidation Reactions of Cyclometalated Palladium(II) and Platinum(II) Complexes

New C,N,S-cyclometalated palladium(II) and platinum(II) complexes have been synthesized and their structural, electrochemical, and photochemical properties examined. The blue color of these complexes in solution changed to yellow under visible-light irradiation. By measurement of the absorption spectra for quantifying changes in color, isosbestic points for each complex clearly indicated the presence of only two species responsible for the change of color. X-ray analysis revealed that the visible-light-induced yellow species were S-oxygenated sulfinato complexes. Photosensitized generation of singlet oxygen (¹O₂) was confirmed by the direct detection of singlet oxygen luminescence at 1275 nm. The present cyclometalated palladium(II) and platinum(II) complexes are efficient photosensitizers of singlet oxygen, which rapidly reacts with coordinating sulfur atoms.

A water-soluble cyclometalated iridium(iii) complex for pH sensing based on aggregation-induced enhanced phosphorescence

The novel water-soluble monoanionic Ir(iii) complex Na[Ir(ppy)2(SB-COO)] (2; Hppy = phenylpyridine; HSB-COOH = 4-carboxylanilinesalicylaldehyde Schiff base), which was obtained by the reaction of the novel Ir(iii) complex [Ir(ppy)2(SB-COOH)] (1) with NaOEt, in its aqueous solution, showed hydrogen ion (H+)-responsive aggregation-induced enhanced phosphorescence (AIEP). Both these complexes exhibited very weak and relatively strong emissions in solution and solid states, respectively. The pH-responsiveness of 2 was evaluated from its emission spectra in aqueous solution in the pH range of 8.7-1.8. Above pH 6, 2 showed weak emission with a maximum at 508 nm. Upon decreasing the pH to 4.7, AIEP with a bathochromic shift to 618 nm was induced by the aggregation of 1, whereby the intensity at 618 nm was increased approximately by 50-fold compared to that at pH 6.0. This enhancement is due to restrictions of the geometrical changes in the six-membered chelate ring of the ancillary ligand (Ir-N-C-C-C-O-) and of the intramolecular rotations in the excited state. The enhanced luminescence originates from spin-forbidden metal-to-ligand-ligand charge transfer (3MLLCT). Below pH 2.8, the emission intensity decreased owing to the decrease in the population of the emissive complex 1 upon dissociation of the ancillary ligand from the Ir(ppy)2 unit.

Cyclometalated iridium-BODIPY ratiometric O2 sensors

Here we introduce a new class of ratiometric O2 sensors for hypoxic environments. Two-component structures composed of phosphorescent cyclometalated Ir(iii) complexes and the well-known organic fluorophore BODIPY have been prepared by the 1 : 1 reaction of bis-cyclometalated iridium synthons with pyridyl-substituted BODIPY compounds. Two different cyclometalating ligands are used, which determine the relative energies of the iridium-centered and BODIPY-centered excited states, and the nature of the linker between iridium and BODIPY also has a small influence on the photoluminescence. Some of the conjugates exhibit dual emission, with significant phosphorescence from the iridium site and fluorescence from the BODIPY, and thus function as ratiometric oxygen sensors. Oxygen quenching experiments demonstrate that as O2 is added the phosphorescence is quenched while the fluorescence is unaffected, with dynamic ranges that are well suited for hypoxic sensing (pO2 < 160 mmHg).

A remarkable phosphorescent sensor for acid–base vapours based on an AIPE-active Ir(iii) complex

A novel AIPE-active neutral mononuclear Schiff base ligand Ir(iii) complex has been synthesized for rapid and reversible phosphorescent sensing of acid–base vapours.

Molecular dyad approaches to the detection and photosensitization of singlet oxygen for biological applications

The principles and prospects of a molecular dyad strategy for photocontrolling biological singlet oxygen are highlighted.

Alkyl chain-modified cyclometalated iridium complexes as tunable anticancer and imaging agents

Five mononuclear cyclometalated iridium complexes [1](PF6)-[5](PF6) were prepared using imidazole-based ligands of varying alkyl chain length. The complexes were characterised by various analytical techniques. The single crystal X-ray structures of [2](PF6), [3](PF6) and [4](PF6) revealed the expected distorted Oh structures around the metal centre; however, the chain length was found to play a crucial role in deciding the overall geometry. Theoretical investigations demonstrated that the HOMOs were mainly contributed by iridium and cyclometalated ligands, whereas the LUMOs were constituted from bpy/phen units. The complexes were found to be luminescent with a moderate emission quantum yield and lifetime in CH3CN. The in vitro growth inhibition assay of the complexes with a shorter alkyl chain ([4]+ and [5]+) displayed higher anticancer activity (IC50 < 0.5 μM) compared to the complexes with a longer alkyl chain ([1]+-[3]+) (IC50 < 30 μM) against human breast cancer (MCF-7) cells. The complexes [4]+ and [5]+ also displayed moderate cancer cell selectivity (∼3 times) over normal breast (MCF-10) cells. The flow cytometry assay and fluorescence microscopy analysis suggested that cellular accumulation was primarily responsible for the variation in anticancer activity. Interestingly, without possessing any anticancer activity or toxicity ((IC50 > 50 μM), the complex [1]+ mainly accumulated near the cell membrane outside the cell and displayed a clear image of the cell membrane. The light microscopy images and western blot analysis reveal that complex [4]+ induced combined apoptosis and paraptosis. Thus, tuning the anticancer activity and cellular imaging property mediated by the alkyl chain would be of great importance and would be useful in anticancer research.

Strong Influence of the Ancillary Ligand over the Photodynamic Anticancer Properties of Neutral Biscyclometalated IrIII Complexes Bearing 2-Benzoazole-Phenolates

In this paper, the synthesis, comprehensive characterization and biological and photocatalytic properties of two series of neutral Ir^III biscyclometalated complexes of general formula [Ir(C^N)₂ (N^O)], where the N^O ligands are 2-(benzimidazolyl)phenolate-N,O (L1, series a) and 2-(benzothiazolyl)phenolate-N,O (L2, series b), and the C^N ligands are 2-(phenyl)pyridinate or its derivatives, are described,. Complexes of types a and b exhibit dissimilar photophysical and biological properties. In vitro cytotoxicity tests conclusively prove that derivatives of series a are harmless in the dark against SW480 cancer cells (colon adenocarcinoma), but express enhanced cytotoxicity versus the same cells after stimulation with UV or blue light. In contrast, complexes of type b show a very high cytotoxic activity in the dark, but low photosensitizing ability. Thus, the ancillary N^O ligand is the main factor in terms of cytotoxic activity both in the dark and upon irradiation. However, the C^N ligands play a key role regarding cellular uptake. In particular, the complex of formula [Ir(dfppy)₂ (L1)] (dfppy=2-(4,6-difluorophenyl)pyridinate) [3 a] has been identified as both an efficient photosensitizer for ¹ O₂ generation and a potential agent for photodynamic therapy. These capabilities are probably related to a combination of its notable cellular internalization, remarkable photostability, high photoluminescence quantum yield, and long triplet excited-state lifetime. Both types of complexes exhibit notable catalytic activity in the photooxidation of thioanisole and S-containing aminoacids with full selectivity.

Crystal structure of fac-{5-[(hexyl-aza-nium-yl)meth-yl]-2-(pyridin-2-yl)phenyl-κ2 N,C 1}bis-[2-(pyridin-2-yl)phenyl-κ2 N,C 1]iridium(III) chloride

The asymmetric unit of the title compound, fac-[Ir(C11H8N)2(C18H24N2)]Cl or fac-[Ir(ppy)2(Hppy-NC6)]Cl, contains two [Ir(ppy)2(ppy-NC6)](H+) cations, two Cl- anions and disordered solvent. In each complex mol-ecule, the IrIII ion is coordinated by two C,N-bidentate 2-(pyridin-2-yl)phenyl ligands and one C,N-bidentate N-[4-(pyridin-2-yl)benz-yl]hexan-1-aminium ligand, leading to a distorted fac-octa-hedral coordination environment. In the crystal, the mol-ecules are linked by N-H⋯Cl, C-H⋯π and π-π inter-actions, forming a three-dimensional supra-molecular structure. The hexyl group of one mol-ecule is disordered over two orientations with a refined occupancy ratio of 0.412 (13):0.588 (13). The acetone and hexane solvent mol-ecules were found to be highly disordered and their contribution to the scattering was masked using the solvent-masking routine smtbx.mask in OLEX2 [Rees et al. (2005 ▸). Acta Cryst. D61, 1299-1301]. These solvent mol-ecules are not considered in the given chemical formula and other crystal data.

Photophysical and Photobiological Properties of Dinuclear Iridium(III) Bis-tridentate Complexes

A series of cationic dinuclear iridium(III) complexes (Ir1-Ir5) bearing terpyridine-capped fluorenyl bridging ligands and different polypyridyl or cyclometalating terminal tridentate ligands were synthesized, characterized, and evaluated for their photophysical and photobiological activities. The influence of the bridging and terminal ligands on the photophysical properties of the complexes was investigated by UV-vis absorption, emission, and transient absorption spectroscopy and simulated by TDDFT calculations. All of the complexes displayed strong bridging-ligand localized visible 1π,π* absorption and red- or near-infrared phosphorescence as well as broad triplet excited-state absorption across both visible and NIR wavelengths. These triplet states were assigned as predominantly 3π,π* for Ir1 (τ = 3.1 μs) and Ir4 (τ = 48 μs) and 3CT (charge transfer) for Ir2, Ir3, and Ir5 (τ = 1.7-2.7 μs). Complexes Ir1-Ir5 acted as in vitro photodynamic therapy (PDT) agents toward human SK-MEL-28 melanoma cells when activated with visible light, with submicromolar photocytotoxicity and phototherapeutic indices ranging from 20 to almost 300. The in vitro PDT effects with visible light did not correlate with singlet oxygen (1O2) quantum yields or DNA photocleaving capacity probed under cell-free conditions. All of the Ir(III) complexes phosphoresced brightly when associated with compromised cells (with or without light treatment) and exhibited photoactivated cellular uptake, highlighting the theranostic potential of this new class of Ir(III) complex photosensitizers.

Design and synthesis of a luminescent iridium complex-peptide hybrid (IPH) that detects cancer cells and induces their apoptosis

Tumor necrosis factor related apoptosis inducing ligand (TRAIL) triggers the cell-extrinsic apoptosis pathway by complexation with its signaling receptors such as death receptors (DR4 and DR5). TRAIL is a C₃-symmetric type II transmembrane protein, consists of three monomeric units. Cyclometalated iridium(III) complexes such as fac-Ir(tpy)₃ (tpy = 2-(4-tolyl)pyridine) also possess a C₃-symmetric structure and are known to have excellent luminescence properties. In this study, we report on the design and synthesis of a C₃-symmetric and luminescent Ir complex-peptide hybrid (IPH), which contains a cyclic peptide that had been reported to bind to death receptor (DR5). The results of MTT assay of Jurkat, K562 and Molt-4 cells with IPH and co-staining experiments with IPH and an anti-DR5 antibody indicate that IPH binds to DR5 and induces apoptosis in a manner parallel to the DR5 expression level. Mechanistic studies of cell death suggest that apoptosis and necrosis-like cell death are differentiated by the position of the hydrophilic part that connects Ir complex and the peptide units. These findings suggest that IPHs could be a promising tool for controlling apoptosis and necrosis by activation of the extra-and intracellular cell death pathway and to develop new anticancer drugs that detect cancer cells and induce their cell death.

Luminescent Iridium Complex-Peptide Hybrids (IPHs) for Therapeutics of Cancer: Design and Synthesis of IPHs for Detection of Cancer Cells and Induction of Their Necrosis-Type Cell Death

Death receptors (DR4 and DR5) offer attractive targets for cancer treatment because cancer cell death can be induced by apoptotic signal upon binding of death ligands such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) with death receptors. Cyclometalated iridium(III) complexes such as fac-Ir(tpy)₃ (tpy = 2-(4-tolyl)pyridine) possess a C₃-symmetric structure like TRAIL and exhibit excellent luminescence properties. Therefore, cyclometalated Ir complexes functionalized with DR-binding peptide motifs would be potent TRAIL mimics to detect cancer cells and induce their cell death. In this study, we report on the design and synthesis of C₃-symmetric and luminescent Ir complex-peptide hybrids (IPHs), which possess cyclic peptide that had been reported to bind DR5. The results of 27 MHz quartz-crystal microbalance (QCM) measurements of DR5 with IPHs and costaining experiments of IPHs and anti-DR5 antibody, suggest that IPHs bind with DR5 and undergo internalization into cytoplasm, possibly via endocytosis. It was also found that IPHs induce slow cell death of these cancer cells in a parallel manner to the DR5 expression level. These results indicate that IPHs may offer a promising tool as artificial luminescent mimics of death ligands to develop a new category of anticancer agents that detect and kill cancer cells.

Increasing the triplet lifetime and extending the ground-state absorption of biscyclometalated Ir(iii) complexes for reverse saturable absorption and photodynamic therapy applications

The synthesis, photophysics, reverse saturable absorption, and photodynamic therapeutic effect of six cationic biscyclometalated Ir(iii) complexes (1-6) with extended π-conjugation on the diimine ligand and/or the cyclometalating ligands are reported in this paper. All complexes possess ligand-localized ¹π,π* absorption bands below 400 nm and charge-transfer absorption bands above 400 nm. They are all emissive in the 500-800 nm range in deoxygenated solutions at room temperature. All complexes exhibit strong and broad triplet excited-state absorption at 430-800 nm, and thus strong reverse saturable absorption for ns laser pulses at 532 nm. Complexes 1-4 are strong reverse saturable absorbers at 532 nm, while complex 6 could be a good candidate as a broadband reverse saturable absorber at 500-850 nm. The degree of π-conjugation of the diimine ligand mainly influences the ¹π,π* transitions in their UV-vis absorption spectra, while the degree of π-conjugation of the cyclometalating ligand primarily affects the nature and energies of the lowest singlet and emitting triplet excited states. However, the lowest-energy triplet excited states for complexes 3-6 that contain the same benzo[i]dipyrido[3,2-a:2',3'-c]phenazine (dppn) diimine ligand but different cyclometalating ligands remain the same as the dppn ligand-localized ³π,π* state, which gives rise to the long-lived, strong excited-state absorption in the visible to the near-IR region. All of the complexes exhibit a photodynamic therapeutic effect upon visible or red light activation, with complex 6 possessing the largest phototherapeutic index reported to date (>400) for an Ir(iii) complex. Interactions with biological targets such as DNA suggest that a novel mechanism of action may be at play for the photosensitizing effect. These Ir(iii) complexes also produce strong intracellular luminescence that highlights their potential as theranostic agents.

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.

Green and Red Fluorescent Dyes for Translational Applications in Imaging and Sensing Analytes: A Dual-Color Flag

Red and green are two of the most-preferred colors from the entire chromatic spectrum, and red and green dyes are widely used in biochemistry, immunohistochemistry, immune-staining, and nanochemistry applications. Selective dyes with green and red excitable chromophores can be used in biological environments, such as tissues and cells, and can be irradiated with visible light without cell damage. This critical review, covering a period of five years, provides an overview of the most-relevant results on the use of red and green fluorescent dyes in the fields of bio-, chemo- and nanoscience. The review focuses on fluorescent dyes containing chromophores such as fluorescein, rhodamine, cyanine, boron-dipyrromethene (BODIPY), 7-nitobenz-2-oxa-1,3-diazole-4-yl, naphthalimide, acridine orange, perylene diimides, coumarins, rosamine, Nile red, naphthalene diimide, distyrylpyridinium, benzophosphole P-oxide, benzoresorufins, and tetrapyrrolic macrocycles. Metal complexes and nanomaterials with these dyes are also discussed.

Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

A series of 2-aryloxazolo[4,5-f][1,10]phenanthroline ligands (N^N ligands) and their cationic iridium(III) complexes (1-11, aryl = 4-NO₂-phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh₂-phenyl (4), 4-NH₂-phenyl (5), pyridin-4-yl (6), naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9), anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of π-conjugation (6-11) on oxazolo[4,5-f][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f][1,10]phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized ¹π,π*/¹ILCT (intraligand charge transfer) transitions, mixed with ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong ¹ILCT/¹LLCT/¹MLCT transitions at 350-450 nm, and broad but very weak ³LLCT/³MLCT absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO₂ or NPh₂ substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.