Molecular Engineering of Platinum(II) Terpyridine Complexes with Tetraphenylethylene-Modified Alkynyl Ligands: Supramolecular Assembly via Pt···Pt and/or π-π Stacking Interactions and the Formation of Various Superstructures

Aggregation-Induced Emission Phosphorescence Featured Au-Ag Coordination Polymer with a Diphosphine N-Heterocyclic Carbene Ligand for Highly Sensitive Detection of Cr(VI)

Luminescent materials with aggregation-induced emission (AIE) characteristics have been recognized as highly selective and sensitive probes for the detection of toxic metal ions in recent years. In this paper, a Au-Ag cluster-based coordination polymer [Au3Ag3(L)2(CN)6(H2O)2]n [1, L = 1,3-bis((diphenylphosphanyl)methyl)-4,5-dihydro-imidazolylidene] was prepared by in situ generation of the diphosphine N-heterocyclic carbene (PCNHCP)-type ligand L in the presence of the corresponding metal salts. Compound 1 exhibited 530 nm phosphorescence under 380 nm excitation with a QY of 6.30% and a lifetime (τ) of 7.14 μs in the solid state. 1 showed good AIE behavior in the mixture of MeOH/H2O while the best aggregation state (fwater = 90%, QY = 6.79%, τ = 6.70 μs) exhibited selective and sensitive emission quenching toward Cr(VI) ions. Ultralow detection limits of 9.7 ppb (w/w) for Cr2O72- and 17.9 ppb (w/w) for CrO42- were achieved.

Platinum complexes with aggregation-induced emission

Transition metal-containing materials with aggregation-induced emission (AIE) have brought new opportunities for the development of biological probes, optoelectronic materials, stimuli-responsive materials, sensors, and detectors. Coordination compounds containing the platinum metal have emerged as a promising option for constructing effective AIE platinum complexes. In this review, we classified AIE platinum complexes based on the number of ligands. We focused on the development and performance of AIE platinum complexes with different numbers of ligands and discussed the impact of platinum ion coordination and ligand structure variation on the optoelectronic properties. Furthermore, this review analyzes and summarizes the influence of molecular geometries, stacking models, and aggregation environments on the optoelectronic performance of these complexes. We provided a comprehensive overview of the AIE mechanisms exhibited by various AIE platinum complexes. Based on the unique properties of AIE platinum complexes with different numbers of ligands, we systematically summarized their applications in electronics, biological fields, etc. Finally, we illustrated the challenges and opportunities for future research on AIE platinum complexes, aiming at giving a comprehensive summary and outlook on the latest developments of functional AIE platinum complexes and also encouraging more researchers to contribute to this promising field.

Antiprotozoal Pt(II) Complexes as Luminophores Bearing Monodentate P/As/Sb-Based Donors: An X-ray Diffractometric, Photoluminescence, and 121Sb-Mössbauer Spectroscopic Study with TD-DFT-Guided Interpretation and Predictive Extrapolation toward Bi

In this study, it is demonstrated that the radiative rate constant of phosphorescent metal complexes can be substantially enhanced using monodentate ancillary ligands containing heavy donor atoms. Thus, the chlorido coligand from a Pt(II) complex bearing a monoanionic tridentate C^N*N luminophore ([PtLCl]) was replaced by triphenylphosphane (PPh3) and its heavier pnictogen congeners (i.e., PnPh3 to yield [PtL(PnPh3)]). Due to the high tridentate-ligand-centered character of the excited states, the P-related radiative rate is rather low while showing a significant boost upon replacement of the P donor by heavier As- and Sb-based units. The syntheses of the three complexes containing PPh3, AsPh3, and SbPh3 were completed by unambiguous characterization of the clean products using exact mass spectrometry, X-ray diffractometry, bidimensional NMR, and 121Sb-Mössbauer spectroscopy (for [PtL(SbPh3)]) as well as steady state and time-resolved photoluminescence spectroscopies. Hence, it was shown that the hybridization defects of the Vth main-group atoms can be overcome by complexation with the Pt center. Notably, the enhancement of the radiative rate constants mediated by heavier coligands was achieved without significantly influencing the character of the excited states. A rationalization of the results was achieved by TD-DFT. Even though the Bi-based homologue was not accessible due to phenylation side reactions, the experimental data allowed a reasonable extrapolation of the structural features whereas the hybridization defects and the excited state properties related to the Bi-species and its phosphorescence rate can be predicted by theory. The three complexes showed an interesting antiprotozoal activity, which was unexpectedly notorious for the P-containing complex. This work could pave the road toward new efficient materials for optoelectronics and novel antiparasitic drugs.

Highly branched and ultrathin Au nanodendrites for reduction catalysis

Two-dimensional (2D) materials have garnered significant attention due to their distinctive physicochemical properties, with 2D noble metal nanodendrites being particularly intriguing in terms of their properties and functional prospects. However, the synthesis of ultrathin and highly branched gold nanodendrites (AuNDs) still poses challenges. In this study, we successfully achieved the synthesis of highly branched 2D AuNDs with a thickness of 4 nm by employing a carboxyl-functionalized C22-tailed surfactant along with the co-directing agent 2-mercaptonicotinic acid (2-MNA). The careful selection of specific thiol molecules such as 2-MNA is crucial for controlling the degree of branching and promoting the formation of ultrathin nanodendrites. Furthermore, we extended this method to synthesize alloy nanodendrites (AuAg NDs and AuCoAg NDs) using a similar approach. Due to their highly branched and ultrathin two-dimensional morphology, these prepared AuNDs exhibit excellent catalytic performance in the model reaction for 4-NP reduction. This thiol-induced synthesis strategy for AuNDs opens up new possibilities for designing other Au nanomaterials with an ultrathin morphology/structure.

Design of molecular sensors and switches based on luminescent ruthenium-terpyridine complexes bearing active methylene and triphenylphosphonium motifs as anion recognition sites: experimental and DFT/TD-DFT investigation

Synthesis, characterization and thorough investigation of the photophysical and electrochemical properties of a new category of emissive homo- and heteroleptic Ru(II)-complexes derived from the [4'-(p-triphenylphosphonium methyl phenyl)-2,2':6',2''-terpyridine]bromide (tpy-PhCH2PPh3Br) ligand have been executed in this work. Incorporation of the PhCH2PPh3+Br- group at the terpyridine motif appropriately adjusts the triplet metal-to-ligand charge transfer (3MLCT) and metal-centered (3MC) excited states so that the complexes luminesce at room temperature (RT) having lifetimes within the range of 6.82-9.63 ns. The RT emission characteristics of the complexes get further enhanced via aggregation phenomena through the use of different solvent/non-solvent mixtures (DMSO/H2O and DMSO/PhCH3 mixtures). Temperature dependent emission spectral measurements indicate that the emission intensity, quantum yield and lifetime increase upon dropping down the temperature, thereby designated as the on-state, while the increase of temperature causes a reduction of the said properties, indicating the off-state and the process is fully reversible. Taking advantage of the active methylene group coupled with a phosphonium motif, anion sensing characteristics of the complexes are investigated systematically in DMSO through the use of various optical channels and spectroscopic tools. The complexes are very much sensitive to fluoride and to a lesser extent acetate and dihydrogen phosphate among the studied anions. In essence, the complexes function as sensors for temperature and fluoride ion. Computational investigations were also executed via density functional theory (DFT) and time-dependent (TD)-DFT to obtain a clear understanding of the electronic structures of the metalloreceptors, appropriate assignment of the spectral bands and their mode of interaction with selected anions.

Induced Self-Assembly and Disassembly of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complexes with Charge Reversal Properties: "Proof-of-Principle" Demonstration of Ratiometric Förster Resonance Energy Transfer Sensing of pH

A series of pH-responsive alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complexes with charge-reversal properties was synthesized, and the supramolecular assemblies between conjugated polyelectrolyte, PFP-OSO₃^-, and [Pt{bzimpy(TEG)₂}{C≡C-C₆H₃-(COOH)₂-3,5}]Cl (1) have been studied using UV-vis absorption, emission, and resonance light scattering (RLS) spectroscopy. An efficient Förster resonance energy transfer (FRET) from PFP-OSO₃^- donor to the aggregated 1 as acceptor with the aid of Pt(II)···Pt(II) interactions has been presented, which leads to a growth of triplet metal-metal-to-ligand charge transfer (³MMLCT) emission in the low-energy red region. The two-component PFP-OSO₃^--1 ensemble was then exploited as a "proof-of-principle" concept strategy for pH sensing by tracking the ratiometric emission changes. With the aid of judicious molecular design on the pH-driven charge-reversal property, the polyelectrolyte-induced self-assembly and the FRET from PFP-OSO₃^- to the platinum(II) aggregates have been modulated. Together with its excellent reversibility and photostability, the extra stability provided by the Pt(II)···Pt(II) and π-π stacking interactions on top of the electrostatic and hydrophobic interactions existing in polyelectrolye-complex assemblies has led to a selective and sensitive pH sensing assay.

Aggregation-Induced Emission-Active Nanostructures: Beyond Biomedical Applications

The discovery of aggregation-induced emission (AIE) phenomenon in 2001 has had a significant impact on materials development across different research disciplines. AIE-active materials have been widely exploited for various applications in optoelectronics, sensing, biomedical, and stimuli-responsive systems, etc. This is made possible by integrating AIE features with other fields of science and engineering, such as nanoscience and nanotechnology. AIE has been extensively employed, particularly for biomedical applications, such as biosensing, bioimaging, and theranostics. However, development of AIE-based nanotechnology for other applications is comparatively less, although there have been increasing research activities in recent years. Given the significance and potential of the marriage between AIE hallmark and nanotechnology in AIE-active materials development, this review article summarizes and showcases the latest research efforts in AIE-based nanomaterials, including nanomaterials synthesis and their nonbiomedical applications, such as sensing, optoelectronics, functional coatings, and stimuli-responsive systems. A perspective on the outlook of AIE-based nanostructured materials and relevant nanotechnology for nonbiomedical applications will be provided, giving an insight into how to design AIE-active nanostructures as well as their applications beyond the biomedical domain.

Synthesis of a platinacycle: determination of the structure and examination of the photophysical properties based on DFT calculations

Platinum-containing macrocycles (platinacycles) have gained attention due to their unique photoelectric properties. In the present study, a novel conjugated platinacycle was synthesized by the dehydrochlorination coupling reaction of a bipyridine dichloroplatinum(II) complex and a 3,6-diethynylcarbazol derivative. The structure of the platinacycle was confirmed by ¹H/¹³C, ¹H-¹H COSY, HMQC, HMBC, DEPT NMR spectroscopies in conjunction with DFT calculations, IR spectroscopy and MALDI-TOF mass spectrometry. The platinacycle exhibited a UV-vis absorption around 540 nm assignable to ligand-ligand charge transfer, and birefringence in DMF, possibly due to alignment of molecules.

Molecular Alignment of Alkynylplatinum(II) 2,6-Bis(benzimidazol-2-yl)pyridine Double Complex Salts and the Formation of Well-Ordered Nanostructures Directed by Pt···Pt and Donor-Acceptor Interactions

A new class of alkynylplatinum(II) bzimpy (bzimpy = bis(benzimidazol-2-yl)pyridine) double complex salts (DCSs) containing dialkoxynaphthalene or pyromellitic diimide moieties on the alkynyl ligand has been reported to display distinct morphological properties compared to their precursor alkynylplatinum(II) complexes, with the capability of being aligned by the directional Pt···Pt and/or π-π stacking interactions. The incorporation of donor and acceptor units on the alkynyl ligands has been found to significantly perturb the alignment of the oppositely charged complex ions in the DCSs to stack in a twisted head-to-head manner, attributed to the additional driving forces of electrostatic and donor-acceptor interactions. The modulation of the Pt···Pt distances and the extent of aggregate formation have been demonstrated by altering the charge matching between the platinum(II) bzimpy moieties and the donor or acceptor moieties on the alkynyl ligand.

Trinuclear Organometallic Pt-Ir-Pt Complexes: Insights into Photophysical Properties, Amino Acid Binding and Protein Sensing

The rational synthesis of trinuclear emissive organometallic complexes with two equivalent platinum(II) centres appended to the ancillary substituted 2,2'-bipyridyl ligand of the cyclometalated iridium(III) centre is reported here. The alkynyl-platinum moiety and cyclometalated iridium(III) centres have been separated through a non-conjugated CH₂ -O-CH₂ linkage. The emission titration with amino acids reveals that the complexes sense free amino acids. The luminescence sensing of BSA is thus attributed to the amino acid sensing ability of the complexes and confirmed by emission anisotropy and Far-UV CD spectral study. The decrease in α-helix in the CD spectra signifies the changes in the secondary structure of protein in presence of the complexes.

Square planar Au(III), Pt(II) and Cu(II) complexes with quinoline-substituted 2,2':6',2″-terpyridine ligands: From in vitro to in vivo biological properties

Cancer is the second leading cause of death worldwide. Cisplatin has challenged cancer treatment; however, resistance and side effects hamper its use. New agents displaying improved activity and more reduced side effects relative to cisplatin are needed. In this work we present the synthesis, characterization and biological activities of three complexes with quinoline-substituted 2,2':6',2″-terpyridine ligand: [Pt(4'-(2-quin)-terpy)Cl](SO₃CF₃) (1), [Au(4'-(2-quin)-terpy)Cl](PF₆)₂·CH₃CN (2) and [Cu(4'-(2-quin)-terpy)Cl](PF₆) (3). The three complexes displayed a high antiproliferative activity in ovarian carcinoma cell line (A2780) and even more noticeable in a colorectal carcinoma cell line (HCT116) following the order 3 > 2 > 1. The complexes IC₅₀ are at least 20 × lower than the IC₅₀ displayed by cisplatin (15.4 μM) in HCT116 cell line while displaying at the same time, much reduced cytotoxicity in a normal dermal fibroblast culture. These cytotoxic activities seem to be correlated with the inclination angles of 2-quin unit to the central pyridine. Interestingly, all complexes can interact with calf-thymus DNA (CT-DNA) in vitro via different mechanisms, although intercalation seems to be the preferred mechanism at least for 2 and 3 at higher concentrations of DNA. Moreover, circular dichroism (CD) data seems to indicate that complex 3, more planar, induces a high destabilization of the DNA double helix (shift from B-form to Z-form). Higher the deviation from planar, the lower the cytotoxicity displayed by the complexes. Cellular uptake may be also responsible for the different cytotoxicity exhibited by complexes with 3 > 2 >1. Complex 2 seems to enter cells more passively while complex 1 and 3 might enter cells via energy-dependent and -independent mechanisms. Complexes 1-3 were shown to induce ROS are associated with the increased apoptosis and autophagy. Moreover, all complexes dissipate the mitochondrial membrane potential leading to an increased BAX/BCL-2 ratio that triggered apoptosis. Complexes 2 and 3 were also shown to exhibit an anti-angiogenic effect by significantly reduce the number of newly formed blood vessel in a CAM model with no toxicity in this in vivo model. Our results seem to suggest that the increased cytotoxicity of complex 3 in HCT116 cells and its potential interest for further translation to pre-clinical mice xenografts might be associated with: 1) higher % of internalization of HCT116 cells via energy-dependent and -independent mechanisms; 2) ability to intercalate DNA and due to its planarity induced higher destabilization of DNA; 3) induce intracellular ROS that trigger apoptosis and autophagy; 4) low toxicity in an in vivo model of CAM; 5) potential anti-angiogenic effect.

In situ formation of tetraphenylethylene nano-structures on microgels inside living cells via reduction-responsive self-assembly

Controlling the assembly of synthetic molecules in living systems is of significance for their adaptive applications. However, it is difficult to achieve, especially for composite self-assemblies, due to the complexity and dynamic change of the intracellular environment, and there exist technical difficulties for the direct visualization of organic and polymer self-assemblies. Herein, we demonstrate a novel strategy for the in situ formation of self-assembled micro-nano composite structures in a cell milieu using reduction-responsive microgels (MGs) as a platform. The MGs were prepared by a templating and crosslinking method using a synthetic amphiphlic polymer as the basic material and porous CaCO3 microparticles as the template. The aggregation-induced emission (AIE) tetraphenylethylene moieties and reduction-labile disulfide bonds in the MGs were employed as the self-assembly building blocks and triggering sites for the intracellular self-assembly, respectively. In the presence of reductive agents such as glutathione, nano-spikes were gradually formed on the MGs. After the MGs were internalized by cells, the in situ formation of microgel/nano-spike composite structures was evidenced by the enhanced fluorescence intensity and was further confirmed by direct transmission electron microscopy observation. This work provides an effective strategy to cope with the challenging task of achieving and probing controlled self-assembly in a cell milieu, leading to new insights into investigating biological self-assembly and promoting the development of micro-/nanomaterials by learning from nature.

Luminescent terpyridine appended geminal bisazide and bistriazoles: multinuclear Pt(II) complexes and AIPE-based DNA detection with the naked eye

We report square planar Pt(ii) complexes as luminescent biosensors for DNA detection in solution. The sensing is attributed to the aggregation induced bright red photoluminescence (AIPE) of the complexes in the presence of DNA that can be seen with the naked eye using only a 360 nm light source. Terpyridine appended luminescent geminal bistriazoles (L1-L4, from geminal bisazide A through azide-alkyne 'click' cycloaddition) with versatile chelating sites were explored for metal coordination and reaction with Pt(dmso)2Cl2 yielding tetranuclear and dinuclear complexes of Pt(ii) with different N∩N ligand environments. Thermally stable gem-bisazide and bistriazoles are hardly reported in the literature and this is the first report of terpyridine appended geminal bisazide and bistriazoles.

Aggregation-induced phosphorescence sensitization in two heptanuclear and decanuclear gold-silver sandwich clusters

The strategy of aggregation-induced emission enhancement (AIEE) has been proven to be efficient in wide areas and has recently been adopted in the field of metal nanoclusters. However, the relationship between atomically precise clusters and AIEE is still unclear. Herein, we have successfully obtained two few-atom heterometallic gold-silver hepta-/decanuclear clusters, denoted Au6Ag and Au9Ag, and determined their structures by X-ray diffraction and mass spectrometry. The nature of the Au^I⋯Ag^I interactions thereof is demonstrated through energy decomposition analysis to be far-beyond typical closed-shell metal-metal interaction dominated by dispersion interaction. Furthermore, a positive correlation has been established between the particle size of the nanoaggregates and the photoluminescence quantum yield for Au6Ag, manifesting AIEE control upon varying the stoichiometric ratio of Au : Ag in atomically-precise clusters.

Steric Engineering of Cyclometalated Pt(II) Complexes toward High-Contrast Monomer-Excimer-Based Mechanochromic and Vapochromic Luminescence

Ligands play a crucial role in the supramolecular photoluminescence properties of Pt(II) square-planar complexes. To improve the luminescence color responses of N∧C∧N cyclometalated Pt(II) complexes to external stimuli such as mechanical stress and chemical vapors, we have conducted a steric engineering of the previous systems 1a-1d [Inorg. Chem. 2017, 56, 4978-4989] by introducing two tert-butyl groups to the tridentate ligand to form complexes 2a-2c. Unlike the "too low" or "too high" steric hindrance of the NCNPt core in 1a-1d, the combined steric effects of the tert-butyl groups at one side and the pentiptycene group at the other side of the NCNPt core in 2b are "just right" for generating as-prepared powders with pure monomer (green) emission or pure excimer (red) emission, depending on the rate of precipitation from solutions. The synergistic steric effects are also beneficial to the solid-state luminescence quantum efficiency (30-36%). As a result of the differences in steric interactions and thus in the relative monomer vs excimer emission intensity, each complex of 2a-2c performs a two-step luminescence mechanochromism and vapochromism with different color patterns. This work provides an intriguing example of steric engineering of Pt(II) complexes toward highly emissive molecular solids with high-contrast mechanochromic and vapochromic luminescence.

Directional Self-Sorting with Cucurbit[8]uril Controlled by Allosteric π-π and Metal-Metal Interactions

To maximize Coulombic interactions, cucurbit[8]uril (CB[8]) typically forms ternary complexes that distribute the positive charges of the pair of guests (if any) over both carbonylated portals of the macrocycle. We present here the first exception to this recognition pattern. Platinum(II) acetylides flanked by 4'-substituted terpyridyl ligands (tpy) form 2:1 complexes with CB[8] in an exclusively stacked head-to-head orientation in a water/acetonitrile mixture. The host encapsulates the pair of tpy substituents, and both positive Pt centers sit on top of each other at the same CB[8] rim, leaving the other rim free of any interaction with the guests. This dramatic charge imbalance between the CB[8] rims would be electrostatically penalizing, were it not for allosteric π-π interactions between the stacked tpy ligands, and possible metal-metal interactions between both Pt centers. When both tpy and acetylides are substituted with aryl units, the metal-ligand complexes form 2:2 assemblies with CB[8] in aqueous medium, and the directionality of the assembly (head-to-head or head-to-tail) can be controlled, both kinetically and thermodynamically.

Aggregation-induced near-infrared emitting platinum(ii) terpyridyl complex: cellular characterisation and lysosome-specific localisation

An aggregation-induced near-infrared emitting terpyridyl Pt(ii) complex with excellent biocompatibility shows high specificity to lysosomes in HeLa cells.

Enhancement of metal–metal interactions inside a large-cavity synthetic host in water

A new host–guest strategy is developed to enhance metal–metal interactions between metal–terpyridyl complexes in aqueous solution.

Click Synthesis of Shape-Persistent Azodendrimers and their Orthogonal Self-Assembly to Nanofibres

The copper(i)-catalyzed azide–alkyne cycloaddition (CuAAC) reaction has been efficiently utilized to synthesize a series of dendrons with amino functionalities. The aminodendrons successfully underwent azodimerization to furnish a series of pyridyl- and phenyl-based azodendrimers with peripheral alkyl or ether side chain substituents. The molecular structures of the azodendrimers were fully assigned using different spectroscopic techniques, such as 1H NMR and 13C NMR, and the molecular weights were determined using MALDI-TOF mass spectrometry. The molecular self-assembly of the azodendrimers was investigated by scanning electron microscopy and transmission electron microscopy, which revealed the formation of highly ordered and uniform self-assembled nanofibres.