The Metal-Metal-to-Ligand Charge Transfer Excited State and Supramolecular Polymerization of Luminescent Pincer PdII -Isocyanide Complexes

Control of Pt···Pt interactions in Pt(II) complex crystals as luminescence sensors for the detection of external stimuli: recent achievements and perspectives

This review summarizes recent advances in the control of the luminescent chromic behavior of Pt(II) complex crystals, focusing on the design strategies of their molecular and packing structures. Square-planar Pt(II) complexes exhibit unique chromic luminescence owing to flexible Pt···Pt interactions, enabling their use as highly sensitive optical sensors to visualize external stimuli and environmental changes. However, the energy of the Pt···Pt interaction is relatively weak, making it difficult to precisely design and control the chromic luminescence based on the Pt···Pt interactions. This review discusses the challenging control of Pt···Pt interactions based on molecular assembly and presents design strategies using hydrogen bonding, halogen interactions, π-π stacking, and ion-pairing and counterion effects, with specific examples for each approach. Future research will focus on expanding analytical approaches, integrating these materials into sensing devices, and exploring synergies between chromic luminescence and other physical properties such as magnetism and conductivity. These advancements are expected to lead to innovative sensing technologies and significant breakthroughs in analytical chemistry and materials science.

Synthesis and Photophysics of the Doubly Cyclometalated Pd(II) Complexes [Pd(C∧N∧C)(L)], L = PPh3, AsPh3, and SbPh3

While Pt(II) complexes containing doubly cyclometalated ligands as tridentate luminophores are well studied, the synthetic accessibility of their Pd(II) counterparts was lacking for a long time. Inspired by a recent report on the synthesis of [Pd(dpp)(PPh3)] involving the C∧N∧C coordination mode (with dpp2- = 2,6-di(phenid-2-yl)pyridine) and following our own work on closely related Pt(II)-based compounds, we produced the series of complexes [Pd(dpp)(PnPh3)] (Pn = P, As, Sb) by optimizing the synthetic procedure and exploring their reactivity in the process. Our study of the electrochemical (cyclic voltammetry) and photophysical (UV-vis absorption and emission, transient absorption (TA) spectroscopy) properties of the Pd(C∧N∧C) complexes represents the first report on their characterization. We observed UV-vis absorption bands down to 450 nm and electrochemical HOMO-LUMO gaps around 3.2 V, which show minimal variation with different PnPh3 coligands. A more pronounced influence of the coligand was observed in time-resolved emission and TA spectroscopy. The highest photoluminescence quantum yield (ΦL) in the series was found for [Pd(dpp)(AsPh3)], reaching 0.06. The interpretation of the spectroscopic data is supported by (TD-)DFT calculations. Additionally, we report structural and spectroscopic data for several dinuclear Pd(II) complexes, including the precursor {[Pd(dppH)(μ-Cl)]}2 and multiple decomposition products of the sensitive compounds [Pd(dpp)(PnPh3)].

Unraveling cation-cation "attraction" in argentophilic interaction in 2,2'-bipydine coordinated silver complex

The nature of argentophilic interaction in the 2,2'-bipyridine-coordinated silver complex, which manifests counterintuitive cation-cation "attraction," is attributed to ligand stacking and solvation effects in the present article. While charged closed-shell transition metal complexes aggregating spontaneously to form oligomers has long been observed experimentally, the interpretation of the nature of so-called metallophilicity is still ongoing. For the dimer [(2,2'-bpy)2Ag]22+, qualitative electrostatic potential, non-covalent interaction, atoms-in-molecules analyses, and quantitative energy decomposition analysis calculations indicate that the electrostatic repulsion between two like formal charges at silver centers can be overcome by long-range dispersion attraction and short-range electronic correlation from ligands. In addition, delocalizing the net charges on silvers over the whole ligands can decrease electrostatic repulsion of metal centers to stabilize oligomers. The vital role of the screening effect of solvent has also been realized in the bound binding of the title system. Overall, this research highlights the importance of ligand stacking to argentophilicity, while d10-d10 attraction of silver centers presents quite little contribution.

Phosphorescent Dinuclear Pd-Pd Emitters in OLED Applications

Four phosphorescent dinuclear Pd-Pd complexes were synthesized by using the clamping ligand N,N'-diphenylformamidine and primary ligands of C∧N, which both have high ligand field strength to enhance the luminescence of the complexes. The single-crystal data confirmed their clamshell structures and revealed Pd-Pd distances between 2.829 and 2.864 Å. In addition, time-dependent density functional theory calculations revealed the phosphorescent nature of the metal-metal-to-ligand charge transfer and ligand-to-ligand charge transfer in these complexes. Moreover, complexes 3 and 4 were used to fabricate an OLED by a vapor phase deposition technique. At an optimal doping concentration of 5%, the 3-based device showed a maximum external quantum efficiency (EQEmax) of 8.79% for CIE coordinates of (0.49, 0.48) and the 4-based device showed an EQEmax of 13.10% for CIE coordinates of (0.61, 0.37).

Dinuclear Cyclometalated Pincer Nickel(II) Complexes with Metal-Metal-to-Ligand Charge Transfer Excited States and Near-Infrared Emission

Facile non-radiative decay of low-lying metal-centered (MC) dd excited states has been well documented to pose a significant obstacle to the development of phosphorescent NiII complexes due to substantial structural distortions between the dd excited state and the ground state. Herein, we prepared a series of dinuclear Ni2 II,II complexes by using strong σ-donating carbene-phenyl-carbene (CNHC Cphenyl CNHC) pincer ligands, and prepared their dinuclear Pt2 II,II and Pd2 II,II analogues. Dinuclear Ni2 II,II complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni-Ni distances of 2.947-3.054 Å and singlet metal-metal-to-ligand charge transfer (1MMLCT) transitions at 500-550 nm. Their 1MMLCT absorption energies are red-shifted relative to the Pt2 II,II and Pd2 II,II analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni2 II,II complexes produces valence-trapped dinuclear Ni2 II,III species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni2 II,II complexes display phosphorescence (τ=2.6-8.6 μs) in the NIR (800-1400 nm) spectral region in 2-MeTHF and in the solid state at 77 K, which is insensitive to π-conjugation of the coordinated [CNHC Cphenyl CNHC] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the 3dd excited state. Studies have found that the dinuclear Ni2 II,II complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.

Visualizing supramolecular assembly behavior, stimulus response, and solid state emission of higher-order Pt2+ aggregates

Here, we present the first instance of a highly efficient red tetramer aggregate with tunable emission based on a cationic platinum(II) complex in conjunction with a silver cluster anion counterpart. This system exhibits multicolor emission response behaviors, which can be conveniently and directly detected through spectroscopic analysis, showcasing intriguing luminescence changes. The self-assembly of Pt⋯, π-π, and hydrogen bonding interactions not only enables an intriguing color adjustment from green to yellow emission, and eventually to red emission, but also demonstrates the co-existence of the monomer, excimer, and aggregation. These phenomena are further accompanied by well-defined nanostructures. The self-assembly process of these structures exhibits an isodesmic growth mechanism, which is dependent on temperature. In this regard, it exhibits potential applicability in multi-mode logic gates that rely on external stimuli such as concentration, solvent, and temperature. The sensitivity of the aggregates towards chemical stimuli combined with their exceptionally bright emission characteristics renders them suitable for diverse applications including solid-state lighting sensing mechanisms and anticounterfeiting measures. The multi-stimuli responsive phosphorescence and self-assembly behaviors of the cationic platinum(II) complex were substantiated by X-ray crystal structure determination, 1H NMR analysis spectroscopic investigations, computational calculations and scanning electron microscopy (SEM) studies.

Thermo-responsive emission induced by different delocalized excited-states in isomorphous Pd(ii) and Pt(ii) one-dimensional chains

The self-assembly of d8 transition metal complexes is essential for the development of optoelectronic and sensing materials with superior photofunctional properties. However, detailed insight into the electronic delocalization of excited states across multiple molecules, particularly in comparing 5d8 (Pt(ii)) and 4d8 (Pd(ii)) systems, remains ambiguous but important. In this study, we have successfully evaluated the differences in the excited-state delocalization and thermal responses of self-assembled Pt(ii) and Pd(ii) complexes. Although the complexes presented herein, K[M(CN)2(dFppy)]·H2O (M = Pt or Pd, dFppy = 2-(4,6-difluorophenyl)pyridinate), are crystallographically isomorphous with similarly short metal⋯metal contacts, only the Pt(ii) complex exhibited thermal equilibria between delocalized excited states, resulting in a drastic thermochromic luminescence with a red-shift of greater than 100 nm. In contrast, the dimeric localized emission from the Pd(ii) complex showed a significant increase in the quantum yield upon cooling, approaching almost unity.

Manipulation of 1D and 2D self-assembly via geometry modulation of adamantane isocyanide Pt(II) complexes

Two cationic luminescent cyclometalated Pt(II) complexes with adamantane-based isocyanide ligands are reported. This work provides important insights for the manipulation of the 1D and 2D self-assembly of Pt(II) complexes by controlling the geometry.

Strategies for Synthesizing Supramolecular Block Copolymers

Over the past decade, controlled supramolecular polymerization has been extensively studied and gradually shifted to supramolecular block copolymerization. Supramolecular block copolymers (BCPs) are considered the holy grail for developing supramolecular materials with new functionalities due to their fascinating structures and ability to introduce diverse functions. From a thermodynamic view to kinetic aspects, great progress has been made in the synthetic strategies of BCPs in the past few years. This Concept summarizes various strategies to realize supramolecular block copolymerization. The focus is on providing researchers with a methodological basis for achieving heterogeneous nucleation-elongation.

Three-Component Multiblock 1D Supramolecular Copolymers of Ir(III) Complexes with Controllable Sequences

Multicomponent supramolecular block copolymers (BCPs) have attracted much attention due to their potential functionalities, but examples of three-component supramolecular BCPs are rare. Herein, we report the synthesis of three-component multiblock 1D supramolecular copolymers of Ir(III) complexes 1-3 by a sequential seeded supramolecular polymerization approach. Precise control over the kinetically trapped species via the pathway complexity of the monomers is the key to the successful synthesis of BCPs with up to 9 blocks. Furthermore, 5-block BCPs with different sequences could be synthesized by changing the addition order of the kinetic species during a sequentially seeded process. The corresponding heterogeneous nucleation-elongation process has been confirmed by the UV/Vis absorption spectra, and each segment of the multiblock copolymers could be characterized by both TEM and SEM. Interestingly, the energy transfer leads to weakened emission of 1-terminated and enhanced emission of 3-terminated BCPs. This study will be an important step in advancing the synthesis and properties of three-component BCPs.

Fluorescence Amplification of Unsaturated Oxazolones Using Palladium: Photophysical and Computational Studies

Weakly fluorescent (Z)-4-arylidene-5-(4H)-oxazolones (1), Φ_PL < 0.1%, containing a variety of conjugated aromatic fragments and/or charged arylidene moieties, have been orthopalladated by reaction with Pd(OAc)₂. The resulting dinuclear complexes (2) have the oxazolone ligands bonded as a C^N-chelate, restricting intramolecular motions involving the oxazolone. From 2, a variety of mononuclear derivatives, such as [Pd(C^N-oxazolone)(O₂CCF₃)(py)] (3), [Pd(C^N-oxazolone)(py)₂](ClO₄) (4), [Pd(C^N-oxazolone)(Cl)(py)] (5), and [Pd(C^N-oxazolone)(X)(NHC)] (6, 7), have been prepared and fully characterized. Most of complexes 3-6 are strongly fluorescent in solution in the range of wavelengths from green to yellow, with values of Φ_PL up to 28% (4h), which are among the highest values of quantum yield ever reported for organometallic Pd complexes with bidentate ligands. This means that the introduction of the Pd in the oxazolone scaffold produces in some cases an amplification of the fluorescence of several orders of magnitude from the free ligand 1 to complexes 3-6. Systematic variations of the substituents of the oxazolones and the ancillary ligands show that the wavelength of emission is tuned by the nature of the oxazolone, while the quantum yield is deeply influenced by the change of ligands. TD-DFT studies of complexes 3-6 show a direct correlation between the participation of the Pd orbitals in the HOMO and the loss of emission through non-radiative pathways. This model allows the understanding of the amplification of the fluorescence and the future rational design of new organopalladium systems with improved properties.

Luminescence and Palladium: The Odd Couple

The synthesis, photophysical properties, and applications of highly fluorescent and phosphorescent palladium complexes are reviewed, covering the period 2018–2022. Despite the fact that the Pd atom appears closely related with an efficient quenching of the fluorescence of different molecules, different synthetic strategies have been recently optimized to achieve the preservation and even the amplification of the luminescent properties of several fluorophores after Pd incorporation. Beyond classical methodologies such as orthopalladation or the use of highly emissive ligands as porphyrins and related systems (for instance, biladiene), new concepts such as AIE (Aggregation Induced Emission) in metallacages or in coordination-driven supramolecular compounds (CDS) by restriction of intramolecular motions (RIM), or complexes showing TADF (Thermally Activated Delayed Fluorescence), are here described and analysed. Without pretending to be comprehensive, selected examples of applications in areas such as the fabrication of lighting devices, biological markers, photodynamic therapy, or oxygen sensing are also here reported.

Controlled Self-assembly of Gold(I) Complexes by Multiple Kinetic Aggregation States with Nonlinear Optical and Waveguide Properties

Introduction of multiple kinetic aggregation states (Aggs) into the self-assembly pathway could bring complexity and flexibility to the self-assemblies, which is difficult to realize due to the delicate equilibria established among different Aggs bonded by weak noncovalent interactions. Here, we describe a series of chiral and achiral d¹⁰ Au^I bis(N-heterocyclic carbene, NHC) complexes, and the achiral complex could undergo self-assembly with multiple kinetic Aggs. Generation of multiple kinetic Aggs was realized by applying chiral or achiral seeds exhibiting large differences in elongation temperatures for their respective cooperative self-assembly processes. We further showed that the chiral Au^I self-assemblies having non-centrosymmetric packing forms exhibit nonlinear optical response of second harmonic generation (SHG), while the SHG signal is absent in the achiral analogue. The crystalline achiral Au^I self-assemblies could function as optical waveguides with strong emission polarization.

Safe, One-Pot, Homogeneous Direct Synthesis of H2O2

Hydrogen peroxide is an environmentally friendly oxidizing agent but current synthetic methods are wasteful. This is a result of the high flammability of H₂/O₂ mixtures and/or the requirement for cocatalysts. In this paper, we report the synthesis of H₂O₂ by means of a homogeneous catalyst, which allows a safe, one-pot synthesis in water, using only H₂ and O₂. This catalyst is capable of removing electrons from H₂, storing them for the reduction of O₂, and then permitting the protonation of the reduced oxygen to H₂O₂. The turnover number (TON) is 910 under an H₂/O₂ (95/5) atmosphere (1.9 MPa) for 12 h at 23 °C, which is the highest of any homogeneous catalyst. Furthermore, we propose a reaction mechanism based on two crystal structures.

Anion-selective "Turn-on" two color phosphorescent probes based on "Pd-Pd" interaction of a series of cyclometallated Palladium (II) complexes induced by a self-assembly in aqueous solution

Herein, three pairs of cationic cyclometallated palladium (II) complexes with different types of C^N ligands, which is non-phosphorescent in aqueous solution, interestingly, they can be utilized as turn-on blue phosphorescent probes selectively for ClO^-, HSO₃^- and CO₃^2-, and turn-on green phosphorescent probe for HSO₃^- in aqueous solution. These different phosphorescent turn-on responses of Pd(II) complexes could be attributed to the degree of coordination and electrostatic interaction between them with specific anion. It suggests that the selectivity towards specific anion of these cyclometallated Pd(II) complexes can be further improved by rationally tuning the structure and enhancing aromaticity of C^N ligand. Our study reveals that these specific species of anions can effectively induce self-assembly of Pd(II) compounds with different C^N ligand based on PdPd interaction, the aggregation and morphology of palladium complex with anion in aqueous media was also investigated by various means of ¹H NMR, UV/Vis, fluorescence spectra, and dynamic light scattering (DLS) analysis. Moreover, transmission electron microscopy (TEM) reveals that nanowires with increased length of diameters of Pd complexes can form in aqueous solution in presence of anions with different high concentration. Furthermore, the cellular uptake and location of Pd2a was also investigated by confocal imaging for the first time. DFT calculation of monomer and dimer of Pd2a was also performed, which is helpful to explain the turn on phosphorescent effect during self-assembly process.

Optical Signal Modulation in Photonic Waveguiding Heteroarchitectures with Continuously Variable Visible-To-Near-Infrared Emission Color

Photonic circuit systems based on optical waveguiding heteroarchitectures have attracted considerable interest owing to their potential to overcome the speed limitation in electronic circuits by modulating the optical signal at the micro- or nanoscale. However, controlling the parameters, including the wavelength and polarization of the light outcoupling, as well as the sequence among different building blocks, remains a key issue. Herein, supramolecular heteroarchitectures made by phosphorescent organometallic complexes of Pt, Pd, Cu, and Au are applied as photonic logic gates that show continuously variable emission colors from 475 to 810 nm, low waveguide losses down to 0.0077 dB µm^-1 , and remarkable excitation-light polarization-dependent photoluminescence with anisotropy ratios up to 0.68. The sequences among Pt, Pd, Au, and Cu building blocks in the heteroarchitectures are controlled by living supramolecular polymerization or crystallization-driven self-assembly synthetic approaches. The results indicate the prospects for using organometallic complexes and supramolecular synthetic approaches to prepare photonic circuit systems with tunable emission color and controllable sequences among different blocks that achieve modulation of the optical signal in the visible-to-near-infrared spectral region.

Luminescence and Length Control in Nonchelated d8 -Metallosupramolecular Polymers through Metal-Metal Interactions

Supramolecular polymers (SPs) of d8 transition metal complexes have received considerable attention by virtue of their rich photophysical properties arising from metal-metal interactions. However, thus far, the molecular design is restricted to complexes with chelating ligands due to their advantageous preorganization and strong ligand fields. Herein, we demonstrate unique pathway-controllable metal-metal-interactions and remarkable 3 MMLCT luminescence in SPs of a non-chelated PtII complex. Under kinetic control, self-complementary bisamide H-bonding motifs induce a rapid self-assembly into non-emissive H-type aggregates (1A). However, under thermodynamic conditions, a more efficient ligand coplanarization leads to superiorly stabilized SP 1B with extended Pt⋅⋅⋅Pt interactions and remarkably long 3 MMLCT luminescence (τ77 K =0.26 ms). The metal-metal interactions could be subsequently exploited to control the length of the emissive SPs using the seeded-growth approach.

Vapor-Phase Living Assembly of π-Conjugated Organic Semiconductors

In contrast to well-studied amphiphilic block copolymers (BCPs) and π-stacked dyes, living assembly of hydrophobic π-conjugated materials has not yet been explored to date. Using a microspacing physical vapor transport (PVT) technique, the prefabricated microrods of organic semiconductors involving 9,10-dicyanoanthracene (DCA, A) or its binary alloy (B) can act as seeds to initiate living homoepitaxial growth from their ends, giving elongated microrods with controlled length. Red-green-red tricolor fluorescent microrod heterostructures with low dispersity are further realized by living heteroepitaxial growth of B microrod blocks on A seed microrod tips. Upon varying the growth sequence of each block, reverse triblock microrods are also accessible. Such a seed-induced living growth is applicable to triblock microrod heterostructures of more binary combinations as well as even more complex penta- and hepta-block heterostructures comprising A and B. By virtue of a convenient vapor-phase growth method, the present work demonstrates the generality of living assembly of π-conjugated materials.

Exploiting halido ligands to control nucleation pathways and Pt⋯Pt interactions in supramolecular co-polymerizations

We exploit halogen effects to tune metal–metal interactions, nucleation pathways and hetero-seeded growth in supramolecular copolymerizations.

Self-assembly of neutral platinum complexes controlled by thermal inputs

In this report, we describe the self-assembly behavior of neutral platinum complexes in toluene. The platinum complexes were seen to form two different types of assemblies depending on the preparation...

Efficient long-range triplet exciton transport by metal-metal interaction at room temperature

Efficient and long-range exciton transport is critical for photosynthesis and opto-electronic devices, and for triplet-harvesting materials, triplet exciton diffusion length ( [[EQUATION]] ) and coefficient ( [[EQUATION]] ) are key parameters in determining their performances. Herein, we observed that PtII and PdII organometallic nanowires exhibit long-range anisotropic triplet exciton LD of 5-7 μm along the M-M direction using direct photoluminescence (PL) imaging technique by low-power continuous wave (CW) laser excitation. At room temperature, via a combined triplet-triplet annihilation (TTA) analysis and spatial PL imaging, an efficient triplet exciton diffusion was observed for the PtII and PdII nanowires with extended close M-M contact, while is absent in nanowires without close M-M contact. Two-dimensional electronic spectroscopy (2DES) and calculations revealed a significant contribution of the delocalized 1/3[dσ*(M-M)→π*] excited state during the exciton diffusion modulated by the M-M distance.

Reductive C(sp3)-C(sp3) homo-coupling of benzyl or allyl halides with H2 using a water-soluble electron storage catalyst

This paper reports the first example of a reductive C(sp3)-C(sp3) homo-coupling of benzyl/allyl halides in aqueous solution by using H2 as an electron source {turnover numbers (TONs) = 0.5-2.3 for 12 h}. This homo-coupling reaction, promoted by visible light, is catalysed by a water-soluble electron storage catalyst (ESC). The reaction mechanism, and four requirements to make it possible, are also described.

Rigid Bridge-Confined Double-Decker Platinum(II) Complexes Towards High-Performance Red and Near-Infrared Electroluminescence

A molecular design to high-performance red and near-infrared (NIR) organic light-emitting diodes (OLEDs) emitters remains demanding. Herein a series of dinuclear platinum(II) complexes featuring strong intramolecular Pt⋅⋅⋅Pt and π-π interactions has been developed by using N-deprotonated α-carboline as a bridging ligand. The complexes in doped thin films exhibit efficient red to NIR emission from short-lived (τ=0.9-2.1 μs) triplet metal-metal-to-ligand charge transfer (³ MMLCT) excited states. Red OLEDs demonstrate high maximum external quantum efficiencies (EQEs) of up to 23.3 % among the best Pt^II -complex-doped devices. The maximum EQE of 15.0 % and radiance of 285 W sr^-1  m^-2 for NIR OLEDs (λ_EL =725 nm) are unprecedented for devices based on discrete molecular emitters. Both red and NIR devices show very small efficiency roll-off at high brightness. Appealing operational lifetimes have also been revealed for the devices. This work sheds light on the potential of intramolecular metallophilicity for long-wavelength molecular emitters and electroluminescence.

Enhanced phosphorescence of Pd(II) and Pt(II) complexes adsorbed onto Laponite for optical sensing of triplet molecular dioxygen in water

Two isoleptic complexes involving Pd(II) and Pt(II) centres with a tetradentate luminophoric ligand were adsorbed onto Laponite (LAP), yielding H2O-dispersible nanohybrids with remarkable photoexcited state properties. The adsorption promoted rigidification of the coordination compounds and suppression of roto-vibrational deactivation pathways, thus enhancing the performance of the molecular species. Interestingly, room temperature phosphorescence was achieved in the case of the Pd(II)-containing nanohybrid along with a marked 3O2 sensitivity, whereas the photoluminescence of the Pt(II)-based material showed only a mild dependence on the presence of 3O2 while reaching up to 37% quantum yield.

Recent progress and future challenges in the supramolecular polymerization of metal-containing monomers

The self-assembly of discrete molecular entities into functional nanomaterials has become a major research area in the past decades. The library of investigated compounds has diversified significantly, while the field as a whole has matured. The incorporation of metal ions in the molecular design of the (supra-)molecular building blocks greatly expands the potential applications, while also offering a promising approach to control molecular recognition and attractive and/or repulsive intermolecular binding events. Hence, supramolecular polymerization of metal-containing monomers has emerged as a major research focus in the field. In this perspective article, we highlight recent significant advances in supramolecular polymerization of metal-containing monomers and discuss their implications for future research. Additionally, we also outline some major challenges that metallosupramolecular chemists (will) have to face to produce metallosupramolecular polymers (MSPs) with advanced applications and functionalities.

Photoluminescence enhancement by controllable aggregation and polymerization of octanuclear gold clusters

Aggregation-induced luminescence behaviors of polynuclear metal clusters are intriguing but still mysterious to date. Herein, we synthesize a series of Au8 clusters with different peripheral ligands to investigate their different supramolecular assembly and distinguishable aggregation-induced luminescent behaviors in solution upon the variation of concentration and solvent polarity. Complex 1a is surrounded by two tetraphenyl ethylene (TPE) moieties, whose dense stacking engenders strong aggregation in solution. Furthermore, the incorporation of the TPE-decorated gold clusters into a polymer backbone promotes molecular restriction within a constrained polymer micelle and thus facilitates efficient emission enhancement of gold clusters. In contrast, the other two gold clusters with small ethynylbenzene and chloride as peripheral ligands exhibit low emission efficiency. The contrastive study showcases how the peripheral ligand arrangements influence assemblies of metal clusters and potentiates such effects in the rational design of cluster-based luminescent materials.

Living supramolecular polymerization of fluorinated cyclohexanes

The development of powerful methods for living covalent polymerization has been a key driver of progress in organic materials science. While there have been remarkable reports on living supramolecular polymerization recently, the scope of monomers is still narrow and a simple solution to the problem is elusive. Here we report a minimalistic molecular platform for living supramolecular polymerization that is based on the unique structure of all-cis 1,2,3,4,5,6-hexafluorocyclohexane, the most polar aliphatic compound reported to date. We use this large dipole moment (6.2 Debye) not only to thermodynamically drive the self-assembly of supramolecular polymers, but also to generate kinetically trapped monomeric states. Upon addition of well-defined seeds, we observed that the dormant monomers engage in a kinetically controlled supramolecular polymerization. The obtained nanofibers have an unusual double helical structure and their length can be controlled by the ratio between seeds and monomers. The successful preparation of supramolecular block copolymers demonstrates the versatility of the approach.

Unraveling Halogen Effects in Supramolecular Polymerization

Halogens play a crucial role in numerous natural processes and synthetic materials due to their unique physicochemical properties and the diverse interactions they can engage in. In the field of supramolecular polymerization, however, halogen effects remain poorly understood, and investigations have been restricted to halogen bonding or the inclusion of polyfluorinated side groups. Recent contributions from our group have revealed that chlorine ligands greatly influence molecular packing and pathway complexity phenomena of various metal complexes. These results prompted us to explore the role of the halogen nature on supramolecular polymerization, a phenomenon that has remained unexplored to date. To address this issue, we have designed a series of archetypal bispyridyldihalogen Pt^II complexes bearing chlorine (1), bromine (2), or iodine (3) and systematically compared their supramolecular polymerization in nonpolar media using various experimental methods and theory. Our studies reveal a remarkably different supramolecular polymerization for the three compounds, which can undergo two competing pathways with either slipped (kinetic) or parallel (thermodynamic) molecular packing. The halogen exerts an inverse effect on the energetic levels of the two self-assembled states, resulting in a single thermodynamic pathway for 3, a transient kinetic species for 2, and a hidden thermodynamic state for 1. This seesaw-like bias of the energy landscape can be traced back to the involvement of the halogens in weak N-H···X hydrogen-bonding interactions in the kinetic pathway, whereas in the thermodynamic pathway the halogens are not engaged in the stabilizing interaction motif but rather amplify solvophobic effects.

Modulation of Metallophilic and π-π Interactions in Platinum Cyclometalated Luminophores with Halogen Bonding

Luminescent cyclometalated complexes [M(C^N^N)CN] (M=Pt, Pd; HC^N^N=pyridinyl- (M=Pt 1, Pd 5), benzyltriazolyl- (M=Pt 2), indazolyl- (M=Pt 3, Pd 6), pyrazolyl-phenylpyridine (M=Pt 4)) decorated with cyanide ligand, have been explored as nucleophilic building blocks for the construction of halogen-bonded (XB) adducts using IC₆ F₅ as an XB donor. The negative electrostatic potential of the CN group afforded CN⋅⋅⋅I noncovalent interactions for platinum complexes 1-3; the energies of XB contacts are comparable to those of metallophilic bonding according to QTAIM analysis. Embedding the chromophore units into XB adducts 1-3⋅⋅⋅IC₆ F₅ has little effect on the charge distribution, but strongly affects Pt⋅⋅⋅Pt bonding and π-stacking, which lead to excited states of MMLCT (metal-metal-to-ligand charge transfer) origin. The energies of these states and the photoemissive properties of the crystalline materials are primarily determined by the degree of aggregation of the luminophores via metal-metal interactions. The adduct formation depends on the nature of the metal and the structure of the metalated ligand, the variation of which can yield dynamic XB-supported systems, exemplified by thermally regulated transition 3↔3⋅⋅⋅IC₆ F₅ .

Ligand-controlled and nanoconfinement-boosted luminescence employing Pt(ii) and Pd(ii) complexes: from color-tunable aggregation-enhanced dual emitters towards self-referenced oxygen reporters

In this work, we describe the synthesis, structural and photophysical characterization of four novel Pd(ii) and Pt(ii) complexes bearing tetradentate luminophoric ligands with high photoluminescence quantum yields (Φ L) and long excited state lifetimes (τ) at room temperature, where the results were interpreted by means of DFT calculations. Incorporation of fluorine atoms into the tetradentate ligand favors aggregation and thereby, a shortened average distance between the metal centers, which provides accessibility to metal-metal-to-ligand charge-transfer (3MMLCT) excimers acting as red-shifted energy traps if compared with the monomeric entities. This supramolecular approach provides an elegant way to enable room-temperature phosphorescence from Pd(ii) complexes, which are otherwise quenched by a thermal population of dissociative states due to a lower ligand field splitting. Encapsulation of these complexes in 100 nm-sized aminated polystyrene nanoparticles enables concentration-controlled aggregation-enhanced dual emission. This phenomenon facilitates the tunability of the absorption and emission colors while providing a rigidified environment supporting an enhanced Φ L up to about 80% and extended τ exceeding 100 μs. Additionally, these nanoarrays constitute rare examples for self-referenced oxygen reporters, since the phosphorescence of the aggregates is insensitive to external influences, whereas the monomeric species drop in luminescence lifetime and intensity with increasing triplet molecular dioxygen concentrations (diffusion-controlled quenching).

Strong metal-metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes

Metallophilicity is defined as the interaction among closed-shell metal centers, the origin of which remains controversial, particularly for the roles of spd orbital hybridization (mixing of the spd atomic orbitals of the metal atom in the molecular orbitals of metal complex) and the relativistic effect. Our studies reveal that at close M-M' distances in the X-ray crystal structures of d8 and d10 organometallic complexes, M-M' closed-shell interactions are repulsive in nature due to strong M-M' Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M-M' Pauli repulsion and repulsive M-M' orbital interaction, and (n + 1)p-nd hybridization suppresses M-M' Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level coupled-cluster singles and doubles with perturbative triples computations and is used to account for the fact that the intermolecular or intramolecular Ag-Ag' distance is shorter than the Au-Au' distance, where a weaker Ag-Ag' Pauli repulsion plays an important role. The experimental studies verify the importance of ligands in intermolecular interactions. Although the M-M' interaction is repulsive in nature, the linear coordination geometry of the d10 metal complex suppresses the L-L' (ligand-ligand) Pauli repulsion while retaining the strength of the attractive L-L' dispersion, leading to a close unsupported M-M' distance that is shorter than the sum of the van der Waals radius (rvdw) of the metal atoms.

Consequences of hidden kinetic pathways on supramolecular polymerization

In recent years, the development of sophisticated analytical tools, kinetic models and sample preparation methods has significantly advanced the field of supramolecular polymerization, where the competition of kinetic vs. thermodynamic processes has become commonplace for a wide range of building blocks. Typically, the kinetic pathways are identified in thermally controlled assembly experiments before they ultimately evolve to the thermodynamic minimum. However, there might be cases where the identification and thus the assessment of the influence of kinetic aggregates is not trivial, making the analysis of the self-assembly processes a hard task. Herein, we demonstrate that "hidden" kinetic pathways can have drastic consequences on supramolecular polymerization processes, to the point that they can even overrule thermodynamic implications. To this end, we analyzed in detail the supramolecular polymerization of a chiral PdII complex 1 that forms two competing aggregates (Agg I and Agg II) of which kinetic Agg II is formed through a "hidden" pathway, i.e. this pathway is not accessible by common thermal polymerization protocols. The hidden pathway exhibits two consecutive steps: first, Agg II is formed in a cooperative process, which subsequently evolves to clustered superstructures driven by rapid kinetics. At standard conditions, Agg II displays an extraordinary kinetic stability (>6 months), which could be correlated to its cooperative mechanism suppressing nucleation of thermodynamic Agg I. Furthermore, the fast kinetics of cluster formation sequester monomers from the equilibria in solution and prevents the system from relaxing into the thermodynamic minimum, thus highlighting the key implications of hidden pathways in governing supramolecular polymerization processes.

Bispicolyamine-Based Supramolecular Polymeric Gels Induced by Distinct Different Driving Forces with and Without Zn2

Metal-coordination polymeric gels are interesting areas as organic/inorganic hybrid supramolecular materials. The bispicolylamine (BPA) based gelator (1) showed excellent gelation with typical fibrillar morphology in acetonitrile. Upon complexing 1 with Zn²⁺, complexes ([1 + Zn + ACN]²⁺ and [1 + zinc trifluoromethanesulfonate (ZnOTf)]⁺) with four coordination numbers were formed, which determine the gel structure significantly. A gel-sol transition was induced, driven by the ratio of the two metal complexes produced. Through nuclear magnetic resonance analysis, the driving forces in the gel formation (i.e., hydrogen-bonding and π-π stacking) were observed in detail. In the absence and the presence of Zn²⁺, the intermolecular hydrogen-bonds and π-π stacking were the primary driving forces in the gel formation, respectively. In addition, the supramolecular gels exhibited a monolayer lamellar structure irrespective of Zn²⁺. Conclusively, the gels' elasticity and viscosity reduced in the presence of Zn²⁺.

The Self-Assembly of a Cyclometalated Palladium Photosensitizer into Protein-Stabilized Nanorods Triggers Drug Uptake In Vitro and In Vivo

Enhanced passive diffusion is usually considered to be the primary cause of the enhanced cellular uptake of cyclometalated drugs because cyclometalation lowers the charge of a metal complex and increases its lipophilicity. However, in this work, monocationic cyclometalated palladium complexes [1]OAc (N^N^C^N) and [2]OAc (N^N^N^C) were found to self-assemble, in aqueous solutions, into soluble supramolecular nanorods, while their tetrapyridyl bicationic analogue [3](OAc)₂ (N^N^N^N) dissolved as isolated molecules. These nanorods formed via metallophilic Pd···Pd interaction and π–π stacking and were stabilized in the cell medium by serum proteins, in the absence of which the nanorods precipitated. In cell cultures, these protein-stabilized self-assembled nanorods were responsible for the improved cellular uptake of the cyclometalated compounds, which took place via endocytosis (i.e., an active uptake pathway). In addition to triggering self-assembly, cyclometalation in [1]OAc also led to dramatically enhanced photodynamic properties under blue light irradiation. These combined penetration and photodynamic properties were observed in multicellular tumor spheroids and in a mice tumor xenograft, demonstrating that protein-stabilized nanoaggregation of cyclometalated drugs such as [1]OAc also allows efficient cellular uptake in 3D tumor models. Overall, serum proteins appear to be a major element in drug design because they strongly influence the size and bioavailability of supramolecular drug aggregates and hence their efficacy in vitro and in vivo.

Dimerization–cyclization reactions of isocyanoaryl-tethered alkylidenecyclobutanes via a triplet biradical mediated process

A triplet biradical mediated dimerization–cyclization reaction of isocyanoaryl-tethered alkylidenecyclobutanes to construct macrocyclic skeletons including dihydroquinoline and quinoline units has been reported.

A guide to supramolecular polymerizations

Supramolecular polymers are non-covalent assemblies of unimeric building blocks connected by secondary interactions and hold great promises due to their dynamic nature.