Cyclometalated Platinum Complexes Research Articles

Combined Nucleophilic and Electrophilic Functionalization to Optimize Blue Phosphorescence in Cyclometalated Platinum Complexes.

Ligand-based functionalization strategies have emerged as powerful approaches to tune and optimize blue phosphorescence, which can involve nucleophilic addition to coordinated ligands or electrophilic functionalization via the coordination of exogenous Lewis acids. Whereas both have been used separately to enhance the photophysical properties of organometallic compounds with high-energy triplet states, in this work, we show that these two strategies can be used together on the same platform. Isocyanide-supported cyclometalated platinum compounds undergo nucleophilic addition with diethylamine to form a strong σ-donor acyclic diaminocarbene-supporting ligand. In a subsequent step, a cyanide ancillary ligand is converted into a more strongly π-acidic isocyanoborate via the coordination of a borane Lewis acid. Importantly, both of these ligand-based functionalization steps improve the quantum yields and lifetimes of the blue-phosphorescent complexes. This synergy results in complexes with photoluminescence quantum yields up to 0.40 for deep blue and 0.75 for sky blue regions and PL lifetimes on the order of 10-5 s.

Turn-on Near-Infrared Phosphorescent Recognition of Anion Based on Self-Assembly of Cyclometalated Platinum Complexes That Induce Oncosis and Monitor Living Cells.

The design of bio-responsive functional molecular materials that can undergo self-assembly to form nanostructures within cells in response to cellular endogenous stimuli and the clarification of their prospective reaction mechanisms are of paramount significance. This work aims to elucidate the spatiotemporal generation of subcellular nanostructures and their influence on cellular functionality. Three sets of cyclometalated platinum complexes have been designed and synthesized as near-infrared phosphorescent turn-on probes for specific anions based on dynamic self-assembly in aqueous solution. The augmentation of the quantity of aromatic rings in the NN bidentate ligand of the complex modifies both the intensity of the intermolecular Pt-Pt interaction and the capacity to generate self-assembled nanowires with near-infrared emission. Besides, we explored the impact of the CN ligand's substituent effect on anion recognition, which revealed that complexes with electron-absorbing F atom substitution exhibit superior selectivity for Br-. These complexes display vivid green turn-on luminescence upon interaction with cellular biomolecules, enabling dynamic monitoring of their subcellular distribution and their interaction on diverse conditions. Furthermore, our complexes were observed to induce oncosis in cancer cells, underscoring the potential of our work in facilitating in vitro diagnosis and developing effective theranostic agents for cancer therapy.

Cationic cyclometalated platinum (II) complexes of metformin with self-assembly and AIE behavior as nuclear and mitochondrial targeted anticancer theragnostic agents

Self-assembly and AIE (Aggregation-induced emission) of cyclometalated platinum (II) complexes have attracted lots of attention. The high hydrophilia and low permeability of metformin (Met) has greatly hindered its further clinical application, herein, our work rationally designed and synthesized a pair of theragnostic cyclometalated platinum (II) - metformin complexes. Pt1 and Pt2 can improve bioactivity of Met significantly, and AIE deep-red emission can be mediated by tuning chemical structure of auxiliary ligand in compounds. Moreover, self-assembly of two Pt (II) complexes was revealed, as well as mitochondria and nuclear targeting ability of Pt2 were newly obtained during an AIE process. Meanwhile, Pt2 exhibits 186- and 53-fold enhanced antiproliferative activity compared with Met and Met-free complex analogue, their anticancer mechanisms were also investigated. Herein, our work provides a novel strategy for designing multi-functional platinum-based anticancer theragnostic agents.

Mechanochromism and circularly polarized luminescence of chiral cyclometalated platinum(II) complexes with different configurations and aggregated forms

Using a new synthesis method, we have prepared two different coordination configurations of chiral cyclometalated N^C^N-coordinated and C^N-coordinated Pt(II) complexes by controlling the Cl− source. Single-crystal X-ray diffraction determines the tridentate and bidentate structures. All N^C^N-coordinated and C^N-coordinated Pt(II) complexes emit green-yellow light in CH2Cl2 solutions. Square-planar N^C^N-Coordinated complexes (−)-1 and (−)-2 show remarkable aggregation-induced emission behaviors in mixed CH2Cl2/n-hexane solutions with the emission quantum yield (Φem) intensified significantly. In addition, the luminescent color could change from green-yellow to red by varying the volumetric fraction of n-hexane. Complex (−)-2 grafted with long alkyl chains could form helical stacking aggregates, inducing enhanced ECD signals and measurable CPL activity. The yellow and orange solids of complex (−)-1 can be separated, and a high-contrast mechanochromic luminescence is found for the yellow form. Furthermore, the potential utility has been attempted in anti-counterfeiting applications based on the yellow solid of (−)-1. This study provides a new design strategy for the preparation of chiral cyclometalated Pt(II) complexes with specific configurations and aggregated forms that have various optical and electronic properties.

A Series of Planar Phosphorescent Cyclometalated Platinum(II) Complexes as New Anticancer Theranostic Agents That Induce Oncosis.

Herein, four planar cyclometalated platinum(II) complexes with a main ligand of enlarged aromatic rings have been assessed as effective anticancer theranostic agents for the first time. With an increased number of aromatic rings in the N∧N ligand, 1a-1d exhibit increased lipophilicity and cytotoxicity selectivity. The intensity of the Pt-Pt interaction of each complex can be indicated by an enhanced near-infrared (NIR) emission in phosphate-buffered saline (PBS), their binding activity with biomolecules of bovine serum albumin (BSA) is accompanied by a vivid turn-on green emission, and the intensity gradually decreased from 1a to 1d, which is consistent with the docking of two complexes with BSA. Both "turn-on" NIR and green emission of 1d can be mainly observed in nuclei of living cell within 24 h, while two phosphorescence traces of 1b were recorded in lysosomes by confocal imaging. Moreover, 1d shows the highest efficiency in inducing oncosis of Hela cells, and the relative process was investigated.

Cyclometallated Platinum(II) Complexes Featuring an Unusual, C^N‐Coordinating Pyridyl‐pyridylidene Ligand and L X Coligands: Synthesis, Structures and Dual Luminescence Behavior

AbstractThanks to an unusual protodemethylation reaction, a series of luminescent cyclometallated platinum(II) complexes can be prepared, which incorporate a rare NC‐chelating, pyridyl‐pyridylidene ligand, in combination with OO‐coordinating acetylacetonate (acac) or NO‐coordinating 2‐picolinate (pic) or 8‐hydroxyquinolate. The acac and pic complexes show unusual dual emission in a frozen glass.A series of cyclometallated platinum(II) complexes incorporating a rare, N^C‐chelating, pyridyl‐pyridylidene ligand are described, in which the coordination sphere is completed by two chlorides or an L X co‐ligand, namely O O‐coordinating acetylacetonate (acac), or N^O‐coordinating 2‐picolinate (pic) or 8‐hydroxyquinolinate. The acac and pic complexes have been structurally characterized in the solid state by single‐crystal X‐ray diffraction. These two complexes display red phosphorescence in the solid state at room temperature. In a frozen glass at 77 K, all four complexes show two broad emission bands that span much of the visible spectrum, apparently from two unequilibrated excited states.

Platinum(II)-Based Optical Probes for Imaging Quadruplex DNA Structures via Phosphorescence Lifetime Imaging Microscopy.

G-quadruplex DNA is a non-canonical structure that forms in guanine-rich regions of the genome. There is increasing evidence showing that G-quadruplexes have important biological functions, and therefore molecular tools to visualise these structures are important. Herein we report on a series of new cyclometallated platinum(II) complexes which, upon binding to G-quadruplex DNA, display an increase in their phosphorescence, acting as switch-on probes. More importantly, upon binding to G-quadruplexes they display a selective and distinct lengthening of their emission lifetime. We show that this effect can be used to selectively visualise these structures in cells using Phosphorescence Lifetime Imaging Microscopy (PLIM).

Platinum(II)-Based Optical Probes for Imaging Quadruplex DNA Structures via Phosphorescence Lifetime Imaging Microscopy.

G-quadruplex DNA is a non-canonical structure that forms in guanine-rich regions of the genome. There is increasing evidence showing that G-quadruplexes have important biological functions, and therefore molecular tools to visualise these structures are important. Herein we report on a series of new cyclometallated platinum(II) complexes which, upon binding to G-quadruplex DNA, display an increase in their phosphorescence, acting as switch-on probes. More importantly, upon binding to G-quadruplexes they display a selective and distinct lengthening of their emission lifetime. We show that this effect can be used to selectively visualise these structures in cells using Phosphorescence Lifetime Imaging Microscopy (PLIM).

Efficient detection of picric acid with aggregation-induced phosphorescent emission neutral cyclometalated platinum(II) complexes in aqueous media

Three neutral cyclometalated platinum(II) complexes have been synthesized and fully characterized. All complexes show aggregation-induced phosphorescent emission properties in THF/H2O. These complexes can detect picric acid efficiently in aqueous media with limits of detection of 0.17, 0.03 and 0.15 μM, respectively. Moreover, the complexes exhibit high selectivity for picric acid in the presence of other competitive compounds and perform well in real water samples. The results show that photo-induced electron transfer is responsible for the luminescence quenching of the complexes.

Synthese und photophysikalische Eigenschaften monometallischer C^C* Platin(II)‐Formamidinatkomplexe mit sterisch anspruchsvollen Liganden

AbstractEine neuartige – bisher als unzugänglich angesehene – Klasse von zyklometallierten Platin(II)‐Komplexen wurde durch eine verbesserte Syntheseroute unter Verwendung von Liganden, die durch Dichtefunktionaltheorieberechnungen vorhergesagt wurden, hergestellt. Basierend auf einem gezielten quantenchemischen Screening wurden drei Modellliganden mit unterschiedlichem sterischem Anspruch ausgewählt und eine Serie von sechs photolumineszenten Platin(II)‐Formamidinatkomplexen mit C^C* zyklometallierten Liganden hergestellt. Der Ligand mit dem geringsten sterischen Anspruch führte zu einem bimetallischen Komplex in zwei isomeren Formen, die voneinander getrennt und durch die entsprechenden Kristallstrukturen bestätigt werden konnten. Wie durch die quantenchemischen Rechnungen vorhergesagt, lieferten die sterisch stärker gehinderten Amidinatliganden die monometallischen Komplexe. Aufgrund des Verlusts der Metall‐Metall‐Wechselwirkungen zeigen diese Komplexe eine signifikante hypsochrome Verschiebung der Emissionswellenlänge. In Abhängigkeit vom zyklometallierten Ligand zeigt diese neue Klasse von Platin(II)‐Emittern grün‐blaue bis rein blaue Phosphoreszenz mit Quantenausbeuten von bis zu 87 % und kurzen Abklingzeiten.

An oxygen-sensitive cyclometalated platinum(II) complex with a triphenylamine modified acetylacetone derivative

A cyclometalated Pt(II) complex [Pt(ppy)(TPA-dpa)] Pt2 with a triphenylamine (TPA) substituted diphenyl-1,3-propanedionate (dpa) as auxiliary ligand was designed and synthesized. The effects of the TPA moiety on the photophysical, electrochemical and oxygen sensing properties of the corresponding Pt(II) complex were systematically investigated compared with non-TPA-modified complexes [Pt(ppy)(dpa)] Pt1 and Pt(ppy)(acac). The maximum emission wavelength of Pt2 exhibits a notable redshift (71 nm) with respect to Pt(ppy)(acac). More importantly, the lifetime (25.52 µs) of Pt2 is significantly prolonged compared with those of Pt1 (0.32 µs) and Pt(ppy)(acac) (2.60 µs). Pt2 is highly sensitive to molecular oxygen when immobilized in an ethyl cellulose film. In addition, the oxygen sensing films have excellent reversibility during the quenching and recovering processes.

Bisisocyanide cyclometallated platinum(II) complexes: synthesis, structure, photophysical properties, and mechanochromic behavior

A series of cyclometallated platinum(II) complexes [Pt(ppy){CNAr}2]X with two isocyanide ligands (Hppy = 2-phenylpyridine, Ar = C6H2-2,4,6-Me3, C6H3-2-Cl-6-Me , C6H3-2,6-Cl2, C6H4-4-NMe2, C6H4-4-Me, C6H4 4-Cl, C6H4-4-Br, C6H4-4-I, C6H4-4-CF3, C6H4-3-CF3; X = BF4, OTf) was synthesized by the reaction of the [{Pt(ppy)Cl}2] dimer with isocyanides (yield 52-70%). The structure of the resulting complexes was determined using mass spectrometry, 1H, 13C{1H}, 195Pt{1H}, 1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, and 1H-13C HMBC NMR spectroscopy in solution and solid-state CP/MAS 13C and 195Pt NMR spectroscopy, IR spectroscopy and X-ray diffraction analysis in the solid phase. The photophysical properties of the obtained complexes in the solid phase and the mechanochromic luminescence behavior were studied. In the solid phase, all synthesized compounds phosphoresce in the green or orange range of visible light, while photoluminescence quantum yields reach 26%. Green phosphors exhibit a reversible mechanochromic luminescence change achieved by mechanical grinding (green to orange) and solvent adsorption (orange to green).

Cyclometalated platinum(II) complexes with acyclic diaminocarbene ligands for OLED application.

A novel series of cyclometalated platinum(II) complexes bearing acyclic diaminocarbene (ADC) ancillary ligands were designed and prepared. Their photophysical properties were systematically studied through experimental and theoretical investigations. All complexes exhibit green phosphorescence with a quantum efficiency of up to 45% in 2 wt% doped PMMA film at room temperature. The complexes are used as light-emitting dopants for organic light-emitting diode (OLED) fabrication. The devices displayed a green emission with a maximum current efficiency of 2.9 cd A-1 and a luminance of 2700 cd m-2. These results show that these cyclometalated platinum(II) complexes can be used as efficient green emitting components of OLED devices.

Luminescent cyclometalated platinum compounds with N, P, and O^O ligands: Density‐functional theory studies and analysis of the anticancer potential

Luminescent platinum cyclometalated complexes are species of interest mainly due to their applications in the optoelectronic and biological fields, especially with regard to their anticancer activity. Given this level of interest, a series of cyclometalated (2‐[2′‐thienyl]pyridinate, thpy and 2‐[2,4‐difluorophenyl]pyridinate, dfppy) platinum complexes with N‐donor, PTA (1,3,5‐triaza‐7‐phosphaadamantane) or chrysin‐derived ligands (incorporating piperidine, HL1, or morpholine, HL2, fragments) were synthesized. The complexes are luminescent with tunable emission wavelengths. Aggregation in solution was observed for [Pt(dfppy)L1], 5. Density‐functional theory (DFT) studies provided descriptions of the highest occupied molecular orbital (HOMO) and least unoccupied molecular orbital (LUMO) characteristics and their influence on the photophysical properties. The orbitals of 5–6 were different in nature to those of 1–4. Time‐dependent DFT (TD‐DFT) calculations showed that for 1–4 the excited states S1 and T1 reflect metal‐to‐ligand charge transfer (MLCT) and ligand‐centered (LC) (C^N) contributions while for 5–6 these states are an LC transition centered on L1 or L2. The speciation in DMSO and DMSO/H2O was evaluated. Biological studies showed that [Pt(thpy)Cl(Hthpy)], 1, [Pt(dfppy)Cl(Hdfppy)], 2, and 5 exert significant cytotoxic activity against human cervical (HeLa) and lung (A549) carcinoma cells. The cytotoxicity of 1 increased 2.84‐fold upon irradiation (blue). Microscopy assays on 5 showed that this compound accumulates in cytoplasmic organelles, preferentially in mitochondria. Mitochondrial metabolism was disrupted by the activity of the complexes, leading to a decline in the adenosine triphosphate (ATP) cellular content. Overall, the results show an alternative anticancer activity for complexes 1, 2, and 5, which could be of great interest for the treatment of tumors with acquired resistance to conventional DNA‐targeted anticancer drugs.

Synthesis, characterization and photophysical properties of mixed ligand cyclometalated platinum(II) complexes containing 2-phenylpyridine and pyridine carboxylic acids

Cyclometalated mixed ligand platinum(II) complexes containing 2-phenylpyridine (ppyH) and pyridine carboxylic acids (PCAs), namely picolinic (2-pyacH), nicotinic (3-pyacH) and isonicotinic (4-pyacH) acid, with the general formula trans-N,N-[Pt(ppy)(PCA)Cl] [PCA = 2-pyacH (2), 3-pyacH (3), 4-pyacH (4)] were synthesized and characterized. The crystal structures of the complexes 3 and 4 showed that the PCAs coordinated in a trans-position towards the orthometalated ppyC, forming hydrogen-bonded homodimers through their carboxylic groups. NMR spectroscopic techniques were used to identify the solution-structural features of the complexes 2–4. The results revealed that the dihedral angle (γ) between the PCA plane and the plane of ppy depends on the carboxylic substitution of the pyridine ring and, in the cases of 3 and 4, the solid-state characteristics were retained. All complexes emit similarly in degassed solution of acetone (475–580 nm) with a very low quantum yield (Φ ∼ 0.05–0.17 %), while in the solid state, a green emission centered at about 530 nm with Φ varying from 3.20–4.86 % is produced.

Synthesis and Characterization of Neutral Cyclometalated Platinum(II) Complexes and their Antibacterial and Cytotoxic Evaluations

AbstractA series of neutral cyclometalated platinum(II) complexes bearing 2,6‐bis(2‐naphthyl)pyridine as a C^N^C tridentate chelating ligand with monodentate pyridyl ligands with different substituents 1–3 have been synthesized via double cyclometalation and ligand displacement reaction. The structural, photophysical, electrochemical and aggregation induced emission (AIE) properties of these neutral platinum(II) complexes were systematically studied. Complexes 1–3 exhibited AIE effects with different emission intensities and colors, in which 1 showed the highest quantum efficiency of 8.6 % under aggregated state, and the aggregates were assembled to ordered spheres. Among the Pt(II) complexes, 1 showed a bactericidal activity against both Staphylococcus aureus (S. aureus) (MIC and MBC=3.13 μg/mL) and methicillin‐resistant S. aureus (MRSA) (MIC and MBC=6.25 μg/mL). Complex 1 did not possess noticeable cytotoxicity to human skin HaCaT keratinocytes. The non‐cytotoxic complex 1 would have a good potential to be used for the antibacterial therapy to combat with S. aureus and MRSA‐infected skin diseases.

High-efficiency circularly polarized emission from liquid-crystalline platinum complexes

Realizing both a high emission efficiency and luminescence dissymmetry factor (glum) in circularly polarized solution processable organic light-emitting diodes (CP-OLEDs) remains a significant challenge. In this contribution, two chiral phosphorescent liquid crystals based on cyclometalated platinum complexes are prepared, in which the chiral s-2-methyl-1‑butyl group is introduced into the cyclometalating ligand and the mesogenic fragment is attached to the periphery of the ancillary ligand. The platinum complexes exhibit both smectic and chiral nematic phases as evidenced by polarized optical microscopy, differential scanning calorimetry and small-angle X-ray diffraction. Remarkably, a high photoluminescent quantum efficiency of over 78% and clear circularly polarized luminescent signal with gPL of about 10–2 are observed for the complexes. Further, solution-processed CP-OLEDs show maximum external quantum efficiencies (EQE) of over 15% and strong circularly polarized electroluminescent signals with a gEL ≈ 10–2. This research demonstrates that both liquid crystallinity and the number of chiral centers play key roles in improving the chiroptical property, paving the way for a new approach for the design of high-efficiency CPL emitters.

Dithienylethene‐Containing Cyclometalated Platinum(II) Complexes with Tunable Photochromic and Photophysical Properties

AbstractA series of cyclometalated platinum(II) complexes with a dithienylethene‐containing N^C and an ancillary O^O ligand have been synthesized and characterized. All the complexes, [Pt(thpy‐DTE)(dtpd)] (1), [Pt(CF3‐thpy‐DTE)(dtpd)] (2) and [Pt(Me‐thpy‐DTE)(dtpd)] (3), exhibit photochromism that can be triggered by photoexcitation at 420 nm into the metal‐to‐ligand charge transfer (MLCT)/intraligand charge transfer (ILCT) band at the visible region, which is accompanied with colour changes from yellow to green, as well as the quenching of red phosphorescence originated from the emissive triplet intraligand (3IL) excited state. The photophysical and photochromic properties can be readily tuned by varying the ligands. Moreover, the fatigue resistance and thermal irreversibility of this series of complexes have also been demonstrated by the representative complex 1 with persistence of the photochromic activity after six photoswitching cycles and a thermal decay half‐live of 4.8 days even at 308 K.

Cyclometalated platinum(II) complexes bearing natural arylolefin and quinolines ligands: Synthesis, characterizations, and in vitro cytotoxicity

Nine new cyclometalated platinum(II) complexes of the formula [PtCl(arylolefin)(Q)] (2, 3) and [Pt(arylolefin)(N,O)] (4–10) (arylolefinH: eugenoxyacetic acid, isopropyl eugenoxyacetate; Q: quinoline; (N,OH): quinolin-8-ol, 2-methylquinolin-8-ol, 5,7-dichloroquinloin-8-ol and quinoline-2-carboxylic acid] were synthesized and fully characterized by EA, ESI mass spectrometry, IR and NMR spectroscopy and single-crystal X-ray diffraction for 3, 7 and 10. The results show that the arylolefin binds with PtIIvia the C5 atom of the phenyl ring and the CC allyl in all the synthesized complexes. Quinoline coordinates with PtIIvia its N atom in 2, 3, while deprotonated (N,OH) coordinates with PtIIvia both the N and O atoms in 4–10. The N atom of Q and (N,O) in all complexes are at the cis position with respect to the allyl group of the arylolefin. The XRD confirms the square-planar coordination of PtII and trans position of the arylolefin and quinoline ligands. The results of in vitro cytotoxicity tests against human cancer cell lines KB and Lu of 2, 4–8 indicate that 4 and 5 exhibit the highest activities against Lu and KB cell lines with the IC50 values of 7.1 and 4.1 µM, respectively, which are approximately 6 and 4 times lower than cisplatin.

Cyclometallated Platinum(II) Complexes with Small Crown Ether Rings: Appropriate Choice of the Bridging Diphosphane to Coordinate Potassium Cations.

This account reports the synthesis and structural characterization of the first cyclometallated platinum(II) complex that coordinates a potassium cation in a sandwich arrangement via two 15-crown-5 ether rings within the same molecule. The cooperation of the two small crown ether moieties allows the entrapment of the non-ideal potassium ion. The reaction of the parent thiosemicarbazone ligand 3,4-(C8H16O5)C6H3C(Me)=NN–(H)C(=S)NHMe, 1, containing the crown ether ring, with K2[PtCl4], or alternatively with PtCl2(DMSO)2, and subsequent treatment with the diphosphanes Ph2PCH2PPh2 (dppm) and Ph2PC(=CH2)PPh2 (vdpp) produced the double nuclear platinacycles 3a, 3b, and 4, probably via formation of the 2a and 2b intermediates. Complex 3a with the K+ cation in a sandwich coordination was slightly mixed with 3b lacking any K+. Alternatively, reaction of 1 with K2[PtCl4] or with PtCl2(DMSO)2 followed by the diphosphane Ph2PC(=CH2)PPh2 (vdpp) only gave the dinuclear phosphane-bridged compound 4; this highlights the importance of choosing the right diphosphane ligand. Density functional theory calculations (B3LYP-D3/LANL2DZ-ECP-6.311++G**) revealed similar affinities for both dppm and vdpp derivatives to coordinate potassium cations. Crystal structure analysis was performed for compounds 3a and 4.