Photophysical Enhancement of Triplet Emitters by Coordination-Driven Self-Assembly

Metallacage-based enhanced PDT strategy for bacterial elimination via inhibiting endogenous NO production

Antibiotics are among the most used weapons in fighting microbial infections and have greatly improved the quality of human life. However, bacteria can eventually evolve to exhibit antibiotic resistance to almost all prescribed antibiotic drugs. Photodynamic therapy (PDT) develops little antibiotic resistance and has become a promising strategy in fighting bacterial infection. To augment the killing effect of PDT, the conventional strategy is introducing excess ROS in various ways, such as applying high light doses, high photosensitizer concentrations, and exogenous oxygen. In this study, we report a metallacage-based PDT strategy that minimizes the use of ROS by jointly using gallium-metal organic framework rods to inhibit the production of bacterial endogenous NO, amplify ROS stress, and enhance the killing effect. The augmented bactericidal effect was demonstrated both in vitro and in vivo. This proposed enhanced PDT strategy will provide a new option for bacterial ablation.

The Search for Efficient True Blue and Deep Blue Emitters: An Overview of Platinum Carbene Acetylide Complexes

Despite significant strides achieved in organic light emitting diode (OLED) based display technologies during the last decade, the search for highly stable and efficient true blue/deep blue emitters continues to remain elusive. During the past decade, emitters with the basic molecular scaffold consisting of Pt(II) acetylides and N-heterocyclic carbene (NHC) ligands have opened interesting opportunities to tune the emission properties with desired chromaticity in the blue and deep blue region. With an aim to achieve the desired CIE coordinates along with low device roll-off efficiencies and satisfactory color purity, a number of variations on the basic molecular fragment have been made. A number of NHC Pt(II) alkyne complexes bearing monodentate, bidentate and tridentate ligands have been synthesized and their photophysical investigations have been reported. Although NHC Pt(II) alkyne complexes have been explored in other areas of applications, much of the investigations have been primarily focused for their interesting emission properties appearing particularly in the shorter wavelength (450-495 nm) part of the electromagnetic spectrum for organic light emitting diode (OLED) devices. In this review, we provide an overview of the investigated NHC Pt(II) acetylide complexes by detailing their synthesis, luminescence properties, performance in devices and future perspectives.

Intramolecular rearrangements guided by adaptive coordination-driven reactions toward highly luminescent polynuclear Cu(i) assemblies

Highly luminescent solid-state Cu₆, Au₂Cu₁₀ and Pt₄Cu₁₁ derivatives are obtained in one step reaction thanks to adaptive coordination-driven supramolecular chemistry using pre-assembled flexible Cu(i) precursors.

Engineering orthogonality in the construction of an alternating rhomboidal copolymer with high fidelity via integrative self-sorting

An alternating rhomboidal copolymer was prepared through the combination of orthogonal self-assembly between metal-coordination and host-guest chemistry as well as integrative self-sorting strategy associated with molecular size and steric effect.

Catalyzed M–C coupling reactions in the synthesis of σ-(pyridylethynyl)dicarbonylcyclopentadienyliron complexes

The reactions between terminal ethynylpyridines, (trimethylsilyl)ethynylpyridines and cyclopentadienyliron dicarbonyl iodide were studied under Pd/Cu-catalyzed conditions to develop a synthetic approach to the σ-alkynyl iron complexes Cp(CO)₂Fe–C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C–R (R = ortho-, meta-, para-pyridyl). Depending on the catalyst and reagents used, the yields of the desired σ-pyridylethynyl complexes varied from 40 to 95%. In some cases the reactions with ortho-ethynylpyridine gave as byproduct the unexpected binuclear FePd μ-pyridylvinylidene complex [Cp(CO)Fe{μ₂-η¹(C_α):η¹(C_α)-κ¹(N)-C_α Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C_β(H)(o-C₅H₄N)}(μ-CO)PdI]. The conditions, catalysts, and reagents that provide the highest yields of the desired σ-pyridylethynyl iron compounds were determined. The methods developed allowed the synthesis of the corresponding σ-4-benzothiadiazolylethynyl complex Cp(CO)₂Fe–CC–(4-C₆H₃N₂S) as well. Eventually, synthetic approaches to σ-alkynyl iron complexes of the type Cp(CO)₂Fe–CC–R (R = ortho-, meta-, para-pyridyl, 4-benzothiadiazol-2,1,3-yl) based on the Pd/Cu-catalyzed cross-coupling reactions were elaborated.

Two approaches were developed for the synthesis of iron σ-pyridylethynyl complexes based on Pd/Cu- and Pd-catalyzed Fe–C coupling reactions.

Multicomponent Porphyrin-Based Tetragonal Prismatic Metallacages and their Photophysical Properties

Multicomponent coordination-driven self-assembly has proved to be a convenient approach to prepare advanced supramolecular coordination complexes (SCCs), especially for those with three-dimensional structures. Herein, we report the preparation of three tetragonal prismatic cages via the self-assembly of Pt(PEt₃)₂(OTf)₂, three different linear dipyridyl ligands and porphyrin-based sodium benzoate ligands. Due to the efficient charge separation in the coordination process of Pt(PEt₃)₂(OTf)₂ with pyridine and carboxylic acid and the directionality of metal-coordination bonds, these cages were prepared in high isolated yields (more than 90%). The absorption and emission properties as well as the singlet oxygen quantum yields of these cages were also studied, showing their potential applications as contrast agents for bio-imaging and photosensitizers for photodynamic therapy.

Understanding the Effects of Coordination and Self-Assembly on an Emissive Phenothiazine

The local environment surrounding luminophores can significantly influence their photophysical properties. Herein, we report the self-assembly of a highly emissive platinum(II)-based metallacage. In order to accommodate the connectivity of the platinum(II) building block used in the self-assembly process, the luminophore-containing building block adopts a highly twisted geometry relative to its free form, leading to the emergence of an emissive transition with a radiative rate constant an order of magnitude higher than that of the free luminophore. This increased rate constant is the primary driver for the 10-fold increase in quantum yield from 4.2% to 40%. Model complexes with platinum or methyl groups bound to the nitrogen were synthesized. These complexes had lower quantum yields (10% and non-emissive, respectively) due mainly to decreases in radiative rate constants. Computational studies were conducted and indicated that the excited state of the ensembles, as well as the model complexes, is a result of charge transfer to the pyridyl groups, in contrast to the free luminophore, which involves the diphenyl sulfone moiety. The differences in quantum yields can be explained by a twist in the chromophore upon coordination of platinum or methylation on the pyridyl group, leading to intersystem crossing to a triplet state. This state then becomes more emissive with the addition of platinum, which increases the radiative rate constant via the heavy atom effect. The formation of a metallacage also decreases the non-radiative rate constant by inhibiting the intramolecular motions of the incorporated luminophore.

A Nona-nuclear Heterometallic Pd3 Pt6 "Donut"-Shaped Cage: Molecular Recognition and Photocatalysis

We report a simple, low-symmetry 2-(1-(pyridine-4-methyl)-1H-1,2,3-triazol-4-yl)pyridine ligand that has both monodentate and bidentate binding sites. With platinum(II) and/or palladium(II) ions, two examples of a new nona-nuclear metallo-assembly have been accessed. These complexes were characterized by NMR spectroscopy, electrospray mass spectrometry (ESI-MS), and in key cases, X-ray crystallography. The cages possess three clefts comprised of planar cationic panels. This structural feature enables the binding of planar aromatic guests such as anthracene. More interestingly, the heterometallic assembly was able to catalyze the light-induced [4+2] cycloaddition of anthracene with singlet oxygen.

Heterometallic Ru-Pt metallacycle for two-photon photodynamic therapy

As an effective and noninvasive treatment of various diseases, photodynamic therapy (PTD) relies on the combination of light, a photosensitizer, and oxygen to generate cytotoxic reactive oxygen species that can damage malignant tissue. Much attention has been paid to covalent modifications of the photosensitizers to improve their photophysical properties and to optimize the pathway of the photosensitizers interacting with cells within the target tissue. Herein we report the design and synthesis of a supramolecular heterometallic Ru-Pt metallacycle via coordination-driven self-assembly. While inheriting the excellent photostability and two-photon absorption characteristics of the Ru(II) polypyridyl precursor, the metallacycle also exhibits red-shifted luminescence to the near-infrared region, a larger two-photon absorption cross-section, and higher singlet oxygen generation efficiency, making it an excellent candidate as a photosensitizer for PTD. Cellular studies reveal that the metallacycle selectively accumulates in mitochondria and nuclei upon internalization. As a result, singlet oxygen generated by photoexcitation of the metallacycle can efficiently trigger cell death via the simultaneous damage to mitochondrial function and intranuclear DNA. In vivo studies on tumor-bearing mice show that the metallacycle can efficiently inhibit tumor growth under a low light dose with minimal side effects. The supramolecular approach presented in this work provides a paradigm for the development of PDT agents with high efficacy.

Crystal structure of bis-(aceto-nitrile-κN)(4,4'-di-tert-butyl-2,2'-bi-pyridine-κ2N,N')platinum(II) bis-(tetra-fluorido-borate) packing as head-to-head dimers

The crystal structure of a platinum(II) supra-molecular building block, [Pt(dbbpy)(NCCH3)2](BF4)2 (dbbpy = 4,4'-di-tert-butyl-2,2'-bi-pyridine, C18H24N2) is an example of a rare head-to-head dimer, even with the bulky tert-butyl groups of the bi-pyridine. This packing motif still enables significant π-π inter-actions between two pyridyl groups, and may result from the close proximity of the tetra-fluorido-borate ions to the platinum(II) complexes, resulting in intra-molecular H⋯F distances between 2.156 and 2.573 Å.

Coordination-Driven Self-Assembly of Ruthenium Polypyridyl Nodes Resulting in Emergent Photophysical and Electrochemical Properties

Ruthenium polypyridyl complexes are among the most studied molecular species for photochemical applications such as light-harvesting and photocatalysis, with [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) serving as an iconic example. We report the use of the [Ru(bpy)₂]²⁺ fragment as a 90° acceptor tecton (M) in coordination-driven self-assembly to synthesize a M₄L₄ metallacycle (L = 4,4'-bipyridine) and a M₆L₄ truncated tetrahedral cage [L = 2,4,6-tris(4-pyridyl)-1,3,5-triazine]. The M₆L₄ cage possesses emergent properties attributed to its unique electronic structure, which results in increased visible-light absorption and an emission band that decays biexponentially with times of 3 and 790 ns. The presence of multiple ruthenium centers in the cage results in multiple Ru^III/II reduction events, with a cathodic shift of the first reduction relative to that of [Ru(bpy)₃]Cl₂ (0.56 V vs 1.05 V). The ligand-centered reduction shifts anodically (-1.29 vs -1.64 V) versus the first bpy reduction observed in the parent [Ru(bpy)₃]Cl₂. The photophysical properties are explained by the existence of two localized charge-transfer states in the cage molecule: one that draws upon the bipyridine π* orbitals and the other upon the 2,4,6-tris(4-pyridyl)-1,3,5-triazine π* orbitals.

Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks

We have prepared a series of M^II₄L₆ tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C₆₀ or C₇₀ fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.

Increasing phosphorescent quantum yields and lifetimes of platinum-alkynyl complexes with extended conjugation

Designing platinum-alkynyl complexes to access a metallo-cumulene resonance form increases the quantum yield via increased rigidity.