Heavy-Metal Ions Removal and Iodine Capture by Terpyridine Covalent Organic Frameworks.

Porous materials are excellent candidates for water remediation in environmental issues. However, it is still a key challenge to design efficient adsorbents for rapid water purification from various heavy metal ions-contaminated wastewater in one step. Here, two robust nitrogen-rich covalent organic frameworks (COFs) bearing terpyridine units on the pore walls by a "bottom-up" strategy are reported. Benefitting from the strong chelation interaction between the terpyridine units and various heavy metal ions, these two terpyridine COFs show excellent removal efficiency and capability for Pb2+, Hg2+, Cu2+, Ag+, Cd2+, Ni2+, and Cr3+ from water. These COFs are shown to remove such heavy metal ions with >90% of contents at one time after the aqueous metal ions mixture is passed through the COF filter. The nitrogen-rich features of the COFs also endow them with the capability of capturing iodine vapors, offering the terpyridine COFs the potential for environmental remediation applications.

NIR-II emissive anionic copper nanoclusters with intrinsic photoredox activity in single-electron transfer.

Ultrasmall copper nanoclusters have recently emerged as promising photocatalysts for organic synthesis, owing to their exceptional light absorption ability and large surface areas for efficient interactions with substrates. Despite significant advances in cluster-based visible-light photocatalysis, the types of organic transformations that copper nanoclusters can catalyze remain limited to date. Herein, we report a structurally well-defined anionic Cu40 nanocluster that emits in the second near-infrared region (NIR-II, 1000-1700 nm) after photoexcitation and can conduct single-electron transfer with fluoroalkyl iodides without the need for external ligand activation. This photoredox-active copper nanocluster efficiently catalyzes the three-component radical couplings of alkenes, fluoroalkyl iodides, and trimethylsilyl cyanide under blue-LED irradiation at room temperature. A variety of fluorine-containing electrophiles and a cyanide nucleophile can be added onto an array of alkenes, including styrenes and aliphatic olefins. Our current work demonstrates the viability of using readily accessible metal nanoclusters to establish photocatalytic systems with a high degree of practicality and reaction complexity.

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.

Synthesis, Characterization, and Resistive Memory Behaviors of Highly Strained Cyclometalated Platinum(II) Nanohoops.

Strained carbon nanohoops exhibit attractive photophysical properties due to their unique π-conjugated structure. However, incorporation of such nanohoops into the pincer ligand of metal complexes has rarely been explored. Herein, a new family of highly strained cyclometalated platinum(II) nanohoops has been synthesized and characterized. Strain-promoted C-H bond activation has been observed during the metal coordination process, and Hückel-Möbius topology and random-columnar packing in the solid state are found. Transient absorption spectroscopy revealed the size-dependent excited state properties of the nanohoops. Moreover, the nanohoops have been successfully employed as active materials in the fabrication of solution-processable resistive memory devices, including the use of the smallest platinum(II) nanohoop for the fabrication of a binary memory, with low switching threshold voltages of ca. 1.5 V, high ON/OFF current ratios, and good stability. These results demonstrate that strain incorporation into the structure can be an effective strategy to fundamentally fine-tune the reactivity, optoelectronic, and resistive memory properties.

Thermally activated delayed fluorescence tetradentate ligand-containing gold(III) complexes with preferential molecular orientation and their application in organic light-emitting devices.

A new class of thermally activated delayed fluorescence (TADF) pyridine-/pyrazine-containing tetradentate C^C^N^N gold(III) complexes have been designed and synthesized. Displaying photoluminescence quantum yields (PLQYs) of up to 0.77 in solid-state thin films, these complexes showed at-least a six-fold increase in the radiative decay rate constant (kr) in toluene upon increasing temperature from 210 to 360 K. Using variable-temperature (VT) ultrafast transient absorption (TA) spectroscopy, the reverse intersystem crossing (RISC) processes were directly observed and the activation parameters were determined, in line with the results of the Boltzmann two-level model fittings, in which the energy separation values between the lowest-lying singlet excited state (S1) and the lowest-lying triplet excited state (T1), ΔE(S1-T1), of these complexes were estimated to be in the range of 0.16-0.18 eV. Through strategic modification of the position of the electron-donating -tBu substituent in the cyclometalating ligand, the permanent dipole moments (PDMs) of these tetradentate gold(III) emitters could be manipulated to enhance their horizontal alignment in the emitting layer of organic light-emitting devices (OLEDs). Consequently, the resulting vacuum-deposited OLEDs demonstrated a 30% increase in the theoretical out-coupling efficiency (ηout), as well as promising electroluminescence (EL) performance with maximum external quantum efficiencies (EQEs) of up to 15.7%.

Assembly of Luminescent Chiral Gold(I)-Sulfido Clusters via Chiral Self-Sorting.

Due to the ubiquity of chirality in nature, chiral self-assembly involving self-sorting behaviors has remained as one of the most important research topics of interests. Herein, starting from a racemic mixture of SEG-based (SEG=SEGPHOS) chlorogold(I) precursors, a unique chiral butterfly-shape hexadecanuclear gold(I) cluster (Au16 ) with different ratios of RSEG and SSEG ligands is obtained via homoleptic and heterochiral self-sorting. More interestingly, by employing different chlorogold(I) precursors of opposite chirality (such as RSEG -Au2 and SBIN -Au2 (BIN=BINAP)), an unprecedented heteroleptic and heterochiral self-sorting strategy has been developed to give a series of heteroleptic chiral decanuclear gold(I) clusters (Au10 ) with propellor-shape structures. Heterochiral and heteroleptic self-sorting have also been observed between enantiomers of homoleptic chiral Au10 clusters to result in the heteroleptic chiral Au10 clusters via cluster-to-cluster transformation. Incorporation of heteroleptic ligands is found to decrease the symmetry from S4 of homoleptic meso Au10 to C2 of heteroleptic chiral Au10 clusters. The chirality has been transferred from the axial chiral ligands and stored in the heteroleptic gold(I) clusters.

Evolution of Polynuclear Gold(I) Sulfido Complexes from Clusters and Cages to Macrocycles.

Two unprecedented tetratriacontanuclear and tetraicosanuclear gold(I) sulfido clusters (denoted as Au34-LMe and Au24-LCbz) with different temperature-induced stimulus-responsive behavior and emission property have been constructed by taking advantage of the judiciously designed bidentate phosphine ligand. Au34-LMe represents the highest nuclearity of the gold(I) sulfido cluster with more than a thousand atoms in the molecule. Octagonal macrocycles based on metal-cluster nodes have been assembled for the first time. The self-assembly and temperature-induced stimulus-responsive processes were monitored by 1H and 31P{1H} NMR spectroscopy, and the identities of the discrete gold(I) complexes were established by single-crystal structural analysis and high-resolution electrospray ionization mass spectrometry data. The steric effects exerted by the substituents on the V-shaped 1,3-bis(diphenylphosphino)benzene ligand have been shown to govern the self-assembly from the 1D cluster and 3D cage to 2D macrocycles. This work not only offers a new strategy to construct and regulate the structure of 2D macrocyclic gold(I) sulfido complexes but also lays the foundation for the future precise design and controlled construction of higher polygonal and cluster-node macrocycles.

A Luminescence Assay in the Red for the Detection of Hydrogen Peroxide and Glucose Based on Metal Coordination Polyelectrolyte-Induced Supramolecular Self-Assembly of Alkynylplatinum(II) Complexes.

A new sensing strategy towards hydrogen peroxide based on metal coordination polyelectrolyte-driven self-assembly of alkynylplatinum(II) 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) complex was demonstrated. The cationic in situ-generated Ag(I)-thiocholine coordination polyelectrolytes were shown to induce the supramolecular self-assembly of anionic low-energy red-emissive alkynylplatinum(II) bzimpy complexes via non-covalent Pt(II)⋅⋅⋅Pt(II), electrostatic and π-π stacking interactions. The presence of hydrogen peroxide was shown to inhibit the formation of coordination polyelectrolytes and the coordination polyelectrolyte-induced self-assembly of platinum(II) complexes. The weakening of Pt(II)⋅⋅⋅Pt(II), electrostatic and π-π stacking interactions was supported by UV-vis absorption, emission, and resonance light scattering (RLS) studies. The present assay was also applied to probe glucose indirectly based on the enzymatic reaction of glucose oxidase on the substrate. Operating in a label-free manner, together with the low-energy red emission and large Stokes shift of alkynylplatinum(II) complexes, these features render the proposed design attractive for biological applications.

Tetradentate C∧C∧N∧N Ligand-Containing Gold(III) Complexes with Orange to Deep-Red Thermally Activated Delayed Fluorescence (TADF) and Their Application in Organic Light-Emitting Devices.

A new class of thermally activated delayed fluorescence (TADF) tetradentate C∧C∧N∧N ligand-containing gold(III) complexes containing acridinyl moieties has been designed and synthesized. These complexes exhibit orange-red to deep-red emission with photoluminescence quantum yields (PLQYs) of up to 0.76 in solid-state thin films. Short excited-state lifetimes of ≤2.0 µs and large radiative decay rate constants (kr) in the order of 105 s-1 have also been found in the complexes. High-performance solution-processed and vacuum-deposited organic light-emitting devices (OLEDs) based on these complexes have been fabricated, demonstrating high maximum external quantum efficiencies (EQEs) of 12.2 and 12.7%, respectively, which are among the best values ever reported for red-emitting gold(III)-based OLEDs. In addition, satisfactory operational half-lifetime (LT50) values of up to 34,058 h have been attained in these red-emitting devices. It is found that the operational stability is strongly dependent on the choice of functional groups on the acridinyl moieties, of which the incorporation of -O- and -S- linkers can effectively prolong the LT50 value by an order of magnitude. The TADF properties of the complexes are substantiated by the hypsochromic shift in emission energies and the remarkable enhancement in the emission intensity upon increasing temperature. The TADF properties have also been supported by temperature-dependent ultrafast transient absorption studies, with the direct observation of reverse intersystem crossing (RISC) and the determination of the activation parameters for the very first time, together with their excited-state dynamics.

[n]Cycloparaphenylene-Pillar[5]arene Bismacrocycles: Their Circularly Polarized Luminescence and Multiple Guest Recognition Properties.

Both pillar[n]arenes (P[n]As) and [n]cycloparaphenylenes ([n]CPPs) play an important role in supramolecular chemistry. Herein, we report the precise synthesis of two multifunctional bismacrocycles [n]CPP-P[5]A by integrating P[5]A into the [n]CPP backbone. The photoluminescence quantum yield (ΦF ) of the bismacrocycles was found to show a dramatic increase relative to the corresponding [n]CPPs. The chiral enantiomers (pR)/(pS)-[8]CPP-P[5]A were successfully isolated by chiral HPLC, and showed promising properties of circularly polarized luminescence (glum ≈0.02). In addition, [n]CPP-P[5]A bismacrocycles are capable of binding pyridinium salts and fullerene derivatives with high affinity and specificity within the two distinct cavities. Transient absorption studies showed that photo-induced electron transfer occurs in [10]CPP-P[5]A⊃C60 complex. Our results suggest that [n]CPP-P[5]A are potentially useful in CPL-active materials, multiple guest recognition and supramolecular polymer preparation.

Machine-Learned Electronically Excited States with the MolOrbImage Generated from the Molecular Ground State.

We present a general machine learning framework for probing the electronic state properties using the novel quantum descriptor MolOrbImage. Each pixel of the MolOrbImage records the quantum information generated by the integration of the physical operator with a pair of bra and ket molecular orbital (MO) states. Inspired by the success of deep convolutional neural networks (NNs) in computer vision, we have implemented the convolutional-layer-dominated MO-NN model. Using the orbital energy and electron repulsion integral MolOrbImages, the MO-NN model achieves promising prediction accuracies against the ADC(2)/cc-pVTZ reference for transition energies to both low-lying singlet [mean absolute error (MAE) < 0.16 eV] and triplet (MAE < 0.14 eV) states. An apparent improvement in the prediction of oscillator strength, which has been shown to be challenging previously, has been demonstrated in this study. Moreover, the transferability test indicates the remarkable extrapolation capacity of the MO-NN model to describe the out of data set systems.

Realization of Long Operational Lifetimes in Vacuum-Deposited Organic Light-Emitting Devices Based on para-Substituted Pyridine Carbazolylgold(III) C

A new series of robust C^C^N carbazolylgold(III) complexes is designed and synthesized through the introduction of inert and sterically bulky oligophenyl substituents on the pyridyl moiety of the cyclometalating ligand. High photoluminescence quantum yields of up to 96% are recorded with these complexes doped in solid-state thin films, and short excited-state lifetimes of 0.3 µs or less in the solid state at room temperature are found. Promising electroluminescence (EL) performances are shown by the vacuum-deposited organic light-emitting devices (OLEDs) based on this series of gold(III) complexes. High external quantum efficiencies of up to 19.5% with efficiency roll-offs of down to 10% at a practical luminance brightness level of 1000 cd m-2 are achieved. More importantly, record-long operational lifetimes (LT50) of up to 470,700 h at 100 cd m-2 are realized, which is currently the highest value among all classes of gold(III) complexes with tridentate pincer ligands. Particularly, by introducing a sterically bulky terphenyl moiety on the reactive site of the pyridine ring, the LT50 value is shown to attain ∼7 times longer half-lifetime than that based on the unsubstituted complex. These unprecedented EL performances and the simple synthetic route in a mercury-free fashion make them promising emitting materials for practical OLEDs toward commercialization.

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.

Synthesis, Electrochemistry, Photophysics, and Photochemistry of a Discrete Tetranuclear Copper(I) Sulfido Cluster.

A tetranuclear copper(I) complex, [Cu4{µ-(Ph2P)2NH}4(µ4-S)](PF6)2 (1), was synthesized. It was found to display intense and long-lived phosphorescence in the solid and solution states. The lowest-energy excited state was assigned as ligand-to-metal charge transfer (LMCT) [S2- → Cu4] mixed with some metal-centered (ds/dp) character. In addition, the phosphorescent state of this complex was found to be quenched by pyridinium acceptors via an oxidative electron-transfer quenching process. An excited-state reduction potential of -1.74 V versus saturated salt calomel electrode was estimated through oxidative quenching studies with a series of structurally related pyridinium acceptors, indicative of its strong reducing power in the excited state. From the transient absorption difference spectrum of the tetranuclear copper(I) sulfido complex and 4-(methoxycarbonyl)-N-methylpyridinium hexafluorophosphate, in addition to the characteristic absorption of the pyridinyl radical at ca. 395 nm, two absorption bands at ca. 500 and 660 nm were also observed. The former was assigned as an LMCT absorption [S2- → Cu4] and the latter as an intervalence charge-transfer transition, associated with the mixed-valence species CuI/CuI/CuI/CuII.

Toward the Design and Construction of Supramolecular Functional Molecular Materials Based on Metal-Metal Interactions.

Supramolecular functional materials represent an emerging class of materials that have been governed by the supramolecular chemistry of self-assembled molecules. Such high-order molecular hierarchies have been stabilized by various kinds of noncovalent intermolecular forces including hydrogen bonding, electrostatic, donor-acceptor, π-π stacking interactions, and dispersion forces. Recently, metal-metal interactions have also emerged as an unconventional type of noncovalent interaction that is unique in the metal complex system for the construction of self-assembled metal-based materials. These metal-metal interactions have further imparted the self-assembled materials with rich spectroscopic functionalities. However, the systematic control of these hierarchical architectures through metal-metal interactions remains challenging. In this Perspective, we aim to stimulate research direction in the field with the utilization of such intriguing and unique directional noncovalent metal-metal interactions as one of the driving forces, highlighting the roles and significance of metal-metal interactions and ultimately facilitating a controlled and rational design and synthesis of metallosupramolecular functional materials with rich spectroscopic properties and huge potential for various applications.

Stimuli-Induced Reversible Transformation between Decanuclear and Pentanuclear Gold(I) Sulfido Complexes.

Decanuclear and pentanuclear gold(I) sulfido complexes of phenanthrene- and dibenzothiophene-based diphosphine ligands were synthesized and characterized. Unprecedented stimuli-induced reversible transformation between decanuclear and pentanuclear gold(I) sulfido complexes was observed, which could be readily monitored by NMR and UV-vis absorption spectroscopy in solution. Remarkably, the decanuclear gold(I) sulfido complex (Au10-LPh) was found to show a highly reversible transformation process, which is stable for over 10 successive cycles in solution. The stimuli-induced reversible transformation behavior of the gold(I) sulfido complexes was found to depend on the P-P bite distance of the bidentate phosphine ligands.

Supramolecular Assembly of Organoplatinum(II) Complexes for Subcellular Distribution and Cell Viability Monitoring with Differentiated Imaging.

The ability to precisely control the subcellular distribution of luminous materials presents unprecedented advantages for understanding cell biology and disease therapy. We introduce a luminescence tool for subcellular distribution imaging and differentiation of live and dead cells, utilizing cationic organoplatinum(II) complexes that exhibit well-defined monomeric to aggregate nanostructures along with concentration-dependent switchable luminescence from green to red due to assembly via PtII ⋅⋅⋅PtII and π-π stacking interactions. One of the complexes was chosen to demonstrate the unique lysosome-to-nucleus subcellular re-distribution and imaging capability in live and dead cells, respectively, which represents the first example to discriminate the subcellular localization of platinum(II) complexes through differential luminescence response. These new findings facilitate the fundamental understanding of self-assembly behaviors of platinum(II) complexes for potential subcellular detection assays.

Carbazolylgold(iii) complexes with thermally activated delayed fluorescence switched on by ligand manipulation as high efficiency organic light-emitting devices with small efficiency roll-offs.

A series of carbazolyl ligands has been designed and synthesized through the integration of various electron-donating and electron-accepting motifs, including electron-donating 4-(diphenylamino)aryl and electron-accepting cyano and diphenylphosphine oxide moieties, for the development of a new class of gold(iii) complexes, where the energies of their triplet intraligand and ligand-to-ligand charge transfer excited states can be manipulated for the activation of thermally activated delayed fluorescence (TADF). Upon excitation, these complexes show high photoluminescence quantum yields of up to 80% in solid-state thin films, with short excited state lifetimes down to 1 µs. Vacuum-deposited and solution-processed organic light-emitting devices based on these complexes demonstrate promising electroluminescence (EL) performance with maximum external quantum efficiencies of 15.0% and 11.7%, respectively, and notably small efficiency roll-off values of less than 1% at the practical luminance brightness level of 1000 cd m-2. These distinct EL performances are believed to be due to the occurrence of multichannel radiative decay pathways via both phosphorescence and TADF that significantly shorten the emission lifetimes and hence reduce the occurrence of the detrimental triplet-triplet annihilation in the gold(iii) complexes.

Solvent-Dependent Supramolecular Host-Guest Assemblies of Platinum(II) Tweezers and a Guest System: From Discrete Molecules to High-Ordered Oligomers.

A series of platinum(II) calix[4]arene-based molecular tweezers was synthesized. The studies of the host-guest association with a charge-neutral cyclometalated platinum(II) complex showed a drastic color change and the turning on of near-infrared emission resulting from Pt⋅⋅⋅Pt and π-π interactions. Control of the host-guest assembly process by varying the solvent composition can lead to a change from discrete host and guest molecules to high-ordered host-guest oligomers with the formation of sheet-like nanostructures, demonstrating a rare example of three-state supramolecular host-guest system with high solubility in solvents of diverse polarity. The change in host-guest assembly behaviors could be probed by drastic color changes from yellow to orange to green. The present study provides insights into the systematic design of solvent-responsive molecular materials using molecular tweezers-directed host-guest assembly, with potential applications in colorimetric sensing of changes in the micro-environment.

Elucidation of the key role of Pt···Pt interactions in the directional self-assembly of platinum(II) complexes.

Here, we report the use of an amphiphilic Pt(II) complex, K[Pt{(O3SCH2CH2CH2)2bzimpy}Cl] (PtB), as a model to elucidate the key role of Pt···Pt interactions in directing self-assembly by combining temperature-dependent ultraviolet-visible (UV-Vis) spectroscopy, stopped-flow kinetic experiments, quantum mechanics (QM) calculations, and molecular dynamics (MD) simulations. Interestingly, we found that the self-assembly mechanism of PtB in aqueous solution follows a nucleation-free isodesmic model, as revealed by the temperature-dependent UV-Vis experiments. In contrast, a cooperative growth is found for the self-assembly of PtB in acetone­water (7:1, vol/vol) solution, which is further verified by the stopped-flow experiments, which clearly indicates the existence of a nucleation phase in the acetone­water (7:1, vol/vol) solution. To reveal the underlying reasons and driving forces for these self-assembly processes, we performed QM calculations and show that the Pt···Pt interactions arising from the interaction between the pz and dz2 orbitals play a crucial role in determining the formation of ordered self-assembled structures. In subsequent oligomer MD simulations, we demonstrate that this directional Pt···Pt interaction can indeed facilitate the formation of linear structures packed in a helix-like fashion. Our results suggest that the self-assembly of PtB in acetone­water (7:1, vol/vol) solution is predominantly driven by the directional noncovalent Pt···Pt interaction, leading to the cooperative growth and the formation of fibrous nanostructures. On the contrary, the self-assembly in aqueous solution forms spherical nanostructures of PtB, which is primarily due to the predominant contribution from the less directional hydrophobic interactions over the directional Pt···Pt and π−π interactions that result in an isodesmic growth.