Photoactivable malachite green-based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2-yl)pyridine complexes

Photo-responsive malachite green moieties have been incorporated into an alkynylplatinum(II) bzimpy system. The photo-caged complexes in acetonitrile solutions exhibit self-assembly properties modulable by photo-removal of the cyano protecting group. Distinct aggregate morphologies, which are facilitated by the non-covalent metal-metal and π-π stacking interactions, have been observed before and after photo-irradiation.

Supramolecular assembly of amphiphilic platinum(ii) Schiff base complexes: diverse spectroscopic changes and nanostructures through rational molecular design and solvent control

A new class of amphiphilic tetradentate platinum(ii) Schiff base complexes has been designed and synthesized. The self-assembly properties by exploiting the potential Pt⋯Pt interactions of amphiphilic platinum(ii) Schiff base complexes in the solution state have been systematically investigated. The presence of Pt⋯Pt interactions has further been supported by computational studies and non-covalent interaction (NCI) analysis of the dimer of the complex. The extent of the non-covalent Pt⋯Pt and π-π interactions could be regulated by a variation of the solvent compositions and the hydrophobicity of the complexes, which is accompanied by attractive spectroscopic and luminescence changes and leads to diverse morphological transformations. The present work represents a rare example of demonstration of directed cooperative assembly of amphiphilic platinum(ii) Schiff base complexes by intermolecular Pt⋯Pt interactions in solution with an in-depth mechanistic investigation, providing guiding principles for the construction of supramolecular structures with desirable properties using platinum(ii) Schiff base building blocks.

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.

Multistimuli-Responsive Properties of Aggregated Isocyanide Cycloplatinated(II) Complexes

Here, we describe the neutral cyclometalated tert-butylisocyanide Pt^II complexes, [Pt(C^∧N)Cl(CNBu^t)] 1, the double salts [Pt(C^∧N)(CNBu^t)₂][Pt(C^∧N)Cl₂] 2, and the cationic complexes [Pt(C^∧N)(CNBu^t)₂]ClO₄ 3 [C∧N = difluorophenylpyridine (dfppy, a), 4-(2-pyridyl)benzaldehyde (ppy-CHO, b)]. A comparative study of the pseudopolymorphs 1a, 1a·CHCl₃, 1b, 1b·0.5Toluene, 1b·0.5PhF, and 3a·0.25CH₂Cl₂ reveals strong aggregation through Pt···Pt and/or π···π stacking interactions to give a variety of distinctive one-dimensional (1D) infinite chains, which modulate the photoluminescent properties. This intermolecular long-range aggregate formation is the main origin of the photoluminescent behavior of 1a and 1b complexes, which exhibit highly sensitive and reversible responses to multiple external stimuli including different volatile organic compounds (VOCs), solvents, temperatures, and pressures, with distinct color and phosphorescent color switching from green to red. Furthermore, complex 1b undergoes supramolecular self-assembly via Pt···Pt and/or π···π interactions into a polymer thin polystyrene (PS) film 10 wt % in response to toluene vapors, and 3a exhibits vapochromic and vapoluminescent behavior. Theoretical simulations on the dimer, trimer, and tetramer models of 1a and 1b have been carried out to get insight into the photophysical properties in the aggregated solid state.

Aggregation and Supramolecular Self-Assembly of Low-Energy Red Luminescent Alkynylplatinum(II) Complexes for RNA Detection, Nucleolus Imaging, and RNA Synthesis Inhibitor Screening

As an important nuclear substructure, the nucleolus has received increasing attention because of its significant functions in the transcription and processing of ribosomal RNA in eukaryotic cells. In this work, we introduce a proof-of-concept luminescence assay to detect RNA and to accomplish nucleolus imaging with the use of the supramolecular self-assembly of platinum(II) complexes. Noncovalent interactions between platinum(II) complexes and RNA can be induced by the introduction of a guanidinium group into the complexes, and accordingly, a high RNA affinity can be achieved. Interestingly, the aggregation affinities of platinum(II) complexes enable them to display remarkable luminescence turn-on upon RNA binding, which is a result of the strengthening of noncovalent Pt(II)···Pt(II) and π-π stacking interactions. The complexes exhibit not only intriguing spectroscopic changes and luminescence enhancement after RNA binding but also specific nucleolus imaging in cells. As compared to fluorescent dyes, the low-energy red luminescence and large Stokes shifts of platinum(II) complexes afford a high signal-to-background autofluorescence ratio in nucleolus imaging. Additional properties, including long phosphorescence lifetimes and low cytotoxicity, have endowed the platinum(II) complexes with the potential for biological applications. Also, platinum(II) complexes have been adopted to monitor the dynamics of the nucleolus induced by the addition of RNA synthesis inhibitors. This capability allows the screening of inhibitors and can be advantageous for the development of antineoplastic agents. This work provides a novel strategy for exploring the application of platinum(II) complex-based cell imaging agents based on the mechanism of supramolecular self-assembly. It is envisaged that platinum(II) complexes can be utilized as valuable probes because of the aforementioned appealing advantages.

Platinum(II) Probes for Sensing Polyelectrolyte Lengths and Architectures

Platinum(II) polypyridine complexes of a square-planar geometry have been used as spectroscopic reporters for quantification of various charged species through non-covalent metal-metal interactions. The characterization of molecular weights and architectures of polyelectrolytes represents a challenging task in polymer science. Here, we report the utilization of platinum(II) complex probes and non-covalent metal-metal interactions for sensing polyelectrolyte lengths and architectures. It is found that the platinum(II) probes can bind to linear polyelectrolytes via electrostatic attractions and give rise to significant spectroscopic changes associated with the formation of metal-metal interactions, and the extent of the spectroscopic changes is found to increase with the lengths of the linear polyelectrolytes. Besides, the platinum(II) probes have been found to co-assemble with the linear polyelectrolytes to form well-defined nanofibers, and the lengths of the linear polyelectrolytes can be directly estimated from the diameter of the nanofibers under transmission electron microscopy observation. Interestingly, upon mixing with the platinum(II) probes, polyelectrolytes with bottlebrush architectures have been found to exhibit larger spectroscopic changes than linear polyelectrolytes with the same chemical composition. Combined with the reported theoretical studies on counterion condensation of polyelectrolytes, the platinum(II) complexes are found to function as spectroscopic probes for sensing the charge densities of the polyelectrolytes with different lengths and diverse architectures. Moreover, platinum(II) probes pre-organized in nanostructured aggregates have been found to intercalate into double-stranded DNA, which are naturally occurring biological polyelectrolytes with helical architectures and intercalation sites, to give significant enhancement of spectroscopic changes when compared to the intercalation of monomeric platinum(II) probes into double-stranded DNA.

Amyloid Protein-Induced Supramolecular Self-Assembly of Water-Soluble Platinum(II) Complexes: A Luminescence Assay for Amyloid Fibrillation Detection and Inhibitor Screening

Amyloid fibrillation has been acknowledged as a hallmark of a number of neurodegenerative ailments such as Alzheimer's disease. Accordingly, efficient detection of amyloid fibrillation will allow for great advances in the field of biomedical applications as well as in achieving early medical diagnosis. In this work, a luminescence assay for the sensitive and specific detection of amyloid fibrillation was developed by using platinum(II) complexes as sensing platforms. Supramolecular self-assembly of platinum(II) complexes was induced upon addition of amyloid, leading to alterations in the spectroscopic and luminescence properties of the complexes. As compared to fluorescent dyes, luminescent platinum(II) complexes exhibit attractive large Stokes shifts, phosphorescence lifetimes in the microsecond to submicrosecond regime, and low-energy red emission after aggregation, which are advantageous to biological imaging. At the same time, the platinum(II) complex adopted herein was found to have high photostability, high selectivity and specificity, and low cytotoxicity. The proposed design is the very first approach to detect amyloid fibrillation through the supramolecular self-assembly of luminescent platinum(II) complexes.

Protamine-Induced Supramolecular Self-Assembly of Red-Emissive Alkynylplatinum(II) 2,6-Bis(benzimidazol-2'-yl)pyridine Complex for Selective Label-Free Sensing of Heparin and Real-Time Monitoring of Trypsin Activity

A label-free detection assay is developed based on the design and synthesis of a new anionic alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine complex with water-soluble pendants. With the aid of electrostatic interaction and noncovalent metal-metal and π-π stacking interactions, protamine is shown to induce supramolecular self-assembly of platinum(II) complexes with drastic UV-vis absorption and red emission changes. On the basis of the strong binding affinity of protamine and heparin, the ensemble has been further employed to probe heparin by monitoring the spectroscopic changes. Other than heparin, this ensemble can also detect the activity of trypsin, which can hydrolyze protamine into fragments, leading to the deaggregation of platinum(II) complexes. By modulation of the self-assembly properties of platinum(II) complexes via real-time UV-vis absorption and emission studies, the reported assay has been demonstrated to be a sensitive and selective detection method for trypsin, as well as trypsin inhibitor screening, which is essential for drug discovery.

A Luminescence Turn-On Assay for Acetylcholinesterase Activity and Inhibitor Screening Based on Supramolecular Self-Assembly of Alkynylplatinum(II) Complexes on Coordination Polymer

A new approach toward acetylcholinesterase (AChE) detection has been demonstrated based on the electrostatic interactions between anionic alkynylplatinum(II) complex molecules and cationic coordination polymer, together with the spectroscopic and emission characteristics of alkynylplatinum(II) complexes upon supramolecular self-assembly. This process involves strengthening of distinct noncovalent Pt(II)···Pt(II) and π-π stacking interactions, which is evidenced by UV-vis absorption, emission, and resonance light scattering results. Such a method has been applied to AChE inhibitor screening, which is important as the demand for AChE inhibitor assays arises along with the drug development for Alzheimer's disease. It affords an emission turn-on response and operates in a continuous and label-free fashion. The low-energy red emission and large Stokes shift of alkynylplatinum(II) complexes are advantageous to biological applications.

Theoretical insights into intermolecular interactions during d8 organometallic self-aggregation

Metallophilicity was once considered as the major driver of aggregation in metal–organic complexes.

Self-assembly of platinum(ii) 6-phenyl-2,2'-bipyridine complexes with solvato- and iono-chromic phenomena

Mono- and di-nuclear organoplatinum(ii) monomers with cyclometalated 6-phenyl-2,2'-bipyridine ligands have been successfully constructed. These systems are capable of displaying intriguing solvato- and iono-chromic phenomena by elaborately manipulating non-covalent PtPt metal-metal and π-π stacking interactions for their self-assembly processes.

Precise Modulation of Molecular Building Blocks from Tweezers to Rectangles for Recognition and Stimuli-Responsive Processes

Alkynylplatinum(II) terpyridine complexes have been increasingly explored since the previous decades, mainly arising from their intriguing photophysical properties and aggregation affinities associated with their extensive Pt(II)···Pt(II) and π-π stacking interactions. Through molecular engineering, one can modulate their fundamental properties and assembly behavior by introduction of various functional groups and structural features. They can therefore serve as ideal candidates to construct metal complex-based molecular architectures to provide an alternative to organic compounds. The metal-based framework can be simultaneously built from predetermined building blocks, giving rise to their well-defined, unique, and discrete natures for molecular recognition. The individual constituents can contribute to molecular architectures with their integrated properties, allowing the manipulation of the various noncovalent intermolecular forces and interactions for selective guest capture. In this Account, our recent progress in the development of these metallomolecular frameworks based on the alkynylplatinum(II) terpyridine system and their recognition properties toward different guest molecules will be presented. Phosphorescent molecular tweezers have been constructed from the alkynylplatinum(II) terpyridine moiety to demonstrate host-guest interactions with cationic, charge-neutral and anionic platinum(II), palladium(II), gold(I), and gold(III) complexes and their binding affinities were found to be perturbed by different metal···metal, π-π and electrostatic interactions. The host-guest assembly process has also resulted in dramatic color changes, together with the turning on of near-IR (NIR) emissions as a result of extensive Pt(II)···Pt(II) interactions. Further work has also been performed to demonstrate that the tweezers can selectively recognize π-surfaces of different planar π-conjugated organic guests. The framework of molecular tweezers has been extended to a double-decker tweezers structure, or a triple-decker structure, which can bind two equivalents of square-planar platinum(II) guests cooperatively to induce a significant color change in solution, representing rare examples of discrete Magnus' green-like salts. By the approaches of structural modifications, we have further modulated the host architecture from molecular tweezers to molecular rectangles. The rectangles have been found to show selective encapsulation of different transition metal complex guests based on the size and steric environment of the host cavity. The molecular rectangles also exhibit reversible host-guest association, in which guest capture and ejection processes can be manipulated by the pH environment, illustrating a potential approach for precise and smart delivery of therapeutic reagents to the slightly more acidic cancer cells.

Long-lived triplet excited state in a platinum(ii) perylene monoimide complex

We report the synthesis and solution based photophysical properties of a new Pt(ii)-terpyridine complex coupled to a perylene monoimide (PMI) chromophoric unit through an acetylene linkage. This structural arrangement resulted in quantitative quenching of the highly fluorescent PMI chromophore by introducing metal character into the lowest energy singlet state, thereby leading to the formation of a long-lived PMI-ligand localized triplet excited state (τ = 8.4 μs). Even though the phosphorescence from this triplet state was not observed, highly efficient quenching of this excited state by dissolved oxygen and the observation of singlet oxygen photoluminescence in the near-IR at 1270 nm initially pointed towards triplet excited state character. Additionally, the coincidence of the excited state absorbance difference spectra from the sensitized PMI ligand using a triplet donor and the Pt-PMI complex provided strong evidence for this triplet state assignment, which was further supported by TD-DFT calculations.

Arginine-Rich Peptide-Induced Supramolecular Self-Assembly of Water-Soluble Anionic Alkynylplatinum(II) Complexes: A Continuous and Label-Free Luminescence Assay for Trypsin and Inhibitor Screening

A water-soluble anionic alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complex has been strategically designed and synthesized to show supramolecular self-assembly with cationic arginine-rich peptides through unique noncovalent Pt(II)···Pt(II) and π-π stacking interactions. Upon introduction of trypsin, the arginine-rich peptides can be hydrolyzed into small fragments and deaggregation of the platinum(II) complex molecules is observed. The aggregation-deaggregation process has been probed by UV-vis absorption, emission, and resonance light scattering (RLS) studies. This platinum(II) complex has been employed for developing a new, continuous and label-free luminescence assay for trypsin as well as for inhibitor screening, and has been successfully applied to detect trypsin in diluted serum solutions.

Multiaddressable molecular rectangles with reversible host-guest interactions: modulation of pH-controlled guest release and capture

A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bis-alkynyl ligands. The studies of the host-guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host-guest interactions are found to be strongly perturbed by metal-metal and π-π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van't Hoff analysis and supported by computational study.

Selective sensing of citrate by a supramolecular ensemble formed by a phenazine copper(i) complex and a perylene diimide derivative

A PET based “off–on” fluorescent sensor for citrate has been developed, displaying low interference by other α-hydroxycarboxylates, dicarboxylates and monosaccharides.

Transformable nanostructures of platinum-containing organosilane hybrids: non-covalent self-assembly of polyhedral oligomeric silsesquioxanes assisted by Pt···Pt and π-π stacking interactions of alkynylplatinum(II) terpyridine moieties

An alkynylplatinum(II) terpyridine complex functionalized with polyhedral oligomeric silsesquioxanes (POSS) moieties has been demonstrated to exhibit self-association behavior to give various distinguishable nanostructures with interesting morphological transformation from rings to rods in response to solvent conditions through the stabilization of Pt···Pt and π-π stacking interactions as well as hydrophobic-hydrophobic interactions. These changes can be systemically controlled by varying the solvent composition and have been studied by (1)H NMR, electron microscopy, UV-vis absorption, and emission spectroscopies.

Anion Binding Properties of Alkynylplatinum(II) Complexes with Amide-Functionalized Terpyridine: Host-Guest Interactions and Fluoride Ion-Induced Deprotonation

Molecular sensors able to detect ions are of interest due to their potential application in areas such as pollutant sequestration. Alkynylplatinum(II) terpyridine complexes with an amide-based receptor moiety have been synthesized and characterized. Their anion binding properties based on host-guest interactions have been examined with the use of UV-vis absorption and emission spectral titration studies. Spectral changes were observed for both complexes upon the addition of spherical and nonspherical anions. Their titration profiles were shown to be in good agreement with theoretical results predicting a 1:1 binding model, and the binding constants were determined from the experimental data. Drastic color changes from yellow to orange-red were observed for one of the complexes upon titration with fluoride (F(-)) ion in acetone. These changes were ascribed to the deprotonation of the amide functionalities induced by F(-) ion, and this was confirmed by the restoration of spectral changes upon addition of trifluoroacetic acid to the F(-) ion-complex mixture as well as by electrospray ionization mass spectrometry (ESI-MS) data.

Supramolecular assembly of metal-ligand chromophores for sensing and phosphorescent OLED applications

The exploration of phosphorescent materials based on transition metal-ligand chromophoric complexes represents an important area of research in molecular materials. The knowledge and fundamental understanding of their photophysical properties offer a possible fine-tuning of their electronic absorption and luminescence properties. The strong propensity of d(8) transition metal compounds to form non-covalent metal···metal interactions facilitates supramolecular assembly and the formation of supramolecular nanostructures with interesting photophysical properties. The introduction of supramolecular assembly with hierarchical complexity involving non-covalent interactions could lead to research dimensions of unlimited possibilities and opportunities. This article briefly summarizes the latest progress on the use of d(8) metal-ligand chromophores as chemosensors, as exemplified by alkynylplatinum(II) complexes with emphasis on supramolecular assembly involving non-covalent interactions, and the recent advances in the utilization of d(8) metal-ligand phosphors, as exemplified by alkynylgold(III) system, for the exploration and development of phosphorescent OLEDs.

Bio-imaging with neutral luminescent Pt(ii) complexes showing metal⋯metal interactions

Self-assembled platinum compounds resulting in stable, highly emissive and long-lived species are reported for cell imaging.

Photofunctional organometallics—from fundamentals to design, assembly and functions

A number of soluble platinum(II) and gold(I) complexes have been designed and synthesized. Their luminescence and photophysical behavior have been studied. Their assembly properties, which were governed by the noncovalent metal–metal interactions, have also been investigated with the introduction of various external stimuli. The UV–vis and emission spectral changes of these complexes, which arose from the modulation of their assembly and disassembly behavior, have demonstrated the establishment of a convenient and sensitive detection strategy for various types of analytes. These studies suggested the potential applications of these metal complexes as detection probes for a diverse range of target substrates.