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

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.

Cyanido-bridged diplatinum(ii) complexes: ligand and solvent effect on aggregation and luminescence

The association of platinum(ii)-based luminophores, which is caused by metal⋯metal and π-π stacking interactions, has been actively exploited in supramolecular construction of photofunctional molecular materials. Herein, we describe a series of bimetallic complexes [{Pt(C^N^/*N)}2(CN)][BAr4F], containing cyanido-bridged cyclometalated Pt(ii) chromophore fragments (HC^N^N = 6-phenyl-2,2'-bipyridine, (benzyltriazolyl)-phenylpyridine, and pyrazolyl-phenylpyridine; HC^N*N = N-pentyl-6-phenyl-N-(pyridin-2-yl)pyridin-2-amine; ^/* denote five/six-membered metallocycles). These compounds are intensely phosphorescent at room temperature showing quantum yields up to 0.73 in solution and 0.62 in the solid state, which are generally higher than those of the mononuclear relatives [Pt(C^N^/*N)(CN)]. The complex cations bearing sterically unhindered -C^N^N ligands readily assemble in solution, reaching the tetrameric species [{Pt(C^N^N)}2(CN)]44+ as suggested by diffusion NMR spectroscopy. The size of the aggregates can be regulated by the concentration, temperature, and polarity of the solvent that allows to alter the emission from green to near-IR. In the solid state, the maximum of low-energy luminescence is shifted up to 912 nm. The results show that photophysical properties of discrete complexes and the intermolecular aggregation can be substantially enhanced by utilizing the rigid bimetallic units giving rise to novel dynamic light emitting Pt(ii) systems.

Recent advances in supramolecular fullerene chemistry

Fullerene chemistry has come a long way since 1990, when the first bulk production of C60 was reported. In the past decade, progress in supramolecular chemistry has opened some remarkable and previously unexpected opportunities regarding the selective (multiple) functionalization of fullerenes and their (self)assembly into larger structures and frameworks. The purpose of this review article is to provide a comprehensive overview of these recent developments. We describe how macrocycles and cages that bind strongly to C60 can be used to block undesired addition patterns and thus allow the selective preparation of single-isomer addition products. We also discuss how the emergence of highly shape-persistent macrocycles has opened opportunities for the study of photoactive fullerene dyads and triads as well as the preparation of mechanically interlocked compounds. The preparation of two- or three-dimensional fullerene materials is another research area that has seen remarkable progress over the past few years. Due to the rapidly decreasing price of C60 and C70, we believe that these achievements will translate into all fields where fullerenes have traditionally (third-generation solar cells) and more recently been applied (catalysis, spintronics).

Solution-state mechanochromic luminescence of Pt(ii)-complexes displayed within micellar aromatic capsules

Mechanochromic luminescence (MCL) is an intrinsic phenomenon in the solid state and thus has been hardly observed in solution so far. Here we report that arylethynyl Pt(ii)-complexes with an NCN-pincer ligand are efficiently encapsulated by micellar aromatic capsules in water, through a simple grinding protocol with bent amphiphiles. When a bent pentamethylbenzene-based amphiphile is employed as an optimized capsule component, the resultant host-guest composite, with an average diameter of ∼4 nm, is obtained in water at room temperature. Notably, the nanocomposite displays strong red emission (Φ = 33%, λmax = 700 nm) derived from MCL via intermolecular Pt(ii)⋯Pt(ii) interactions even under aerobic aqueous conditions, in sharp contrast to the free Pt(ii)-complex with weak green emission (Φ = 4%, λmax = 500 nm) in CH2Cl2. Moreover, enhancement of the solution-state MCL (up to Φ = 48%) can be achieved by coencapsulation of the Pt(ii)-complexes with carbazole derivatives by the capsule in water. This study provides the first example of "solution-state" mechanochromic luminescence, capable of facilely tuning its intensity and wavelength, among the intensive studies of various solid-state MCL reported previously.

Sterically attenuated electronic communication in cobalt complexes of meridional isoquinoline-derived ligands for applications in electrocatalysis

The ability to synthetically tune the ligand frameworks of redox-active molecules is of critical importance to the economy of solar fuels because manipulating their redox properties can afford control over the operating potentials of sustained electrocatalytic or photoelectrocatalytic processes. The electronic and steric properties of 2,2':6',2″-terpyridine (Terpy) ligand frameworks can be tuned by functional group substitution on ligand backbones, and these correlate strongly to their Hammett parameters. The synthesis of a new series of tridentate meridional ligands of 2,4,6-trisubstituted pyridines that engineers the ability to finely tune the redox potentials of cobalt complexes to more positive potentials than that of their Terpy analogs is achieved by aryl-functionalizing at the four-position and by including isoquinoline at the two- and six-positions of pyridine (Aryl-DiQ). Their cobalt complex syntheses, their electronic properties, and their catalytic activity for carbon dioxide (CO2) reduction are reported and compared to their Terpy analogs. The cobalt derivatives generally experience a positive shift in their redox features relative to the Terpy-based analogs, covering a complementary potential range. Although those evaluated fail to produce any quantifiable products for the reduction of CO2 and suffer from long-term instability, these results suggest possible alternate strategies for stabilizing these compounds during catalysis. We speculate that lower equilibrium association constants to the cobalt center are intrinsic to these ligands, which originate from a steric interaction between protons on the pyridine and isoquinoline moieties. Nevertheless, the new Aryl-DiQ ligand framework has been engineered to selectively tune homoleptic cobalt complexes' redox potentials.

Order-order assembly transition-driven polyamines detection based on iron-sulfur complexes

Innovative modes of response can greatly push forward chemical sensing processes and subsequently improve sensing performance. Classical chemical sensing modes seldom involve the transition of a delicate molecular assembly during the response. Here, we display a sensing mode for polyamine detection based on an order-order transition of iron-sulfur complexes upon their assembly. Strong validation proves that the unique order-order transition of the assemblies is the driving force of the response, in which the polyamine captures the metal ion of the iron-sulfur complex, leading it to decompose into a metal-polyamine product, accompanied by an order-order transition of the assemblies. This mechanism makes the detection process more intuitive and selective, and remarkably improves the detection efficiency, achieving excellent polyamines specificity, second-level response, convenient visual detection, and good recyclability of the sensing system. Furthermore, this paper also provides opportunities for the further application of the iron-sulfur platform in environment-related fields.

Hourglass-Shaped Nanocages with Concaved Structures Based on Selective Self-Complementary Coordination Ligands and Tunable Hierarchical Self-Assembly

Three-dimensional (3D) structures constructed via coordination-driven self-assemblies have recently garnered increasing attention due to the challenges in structural design and potential applications. In particular, developing new strategy for the convenient and precise self-assemblies of 3D supramolecular structures is of utmost interest. Introducing the concept of self-coordination ligands, herein the design and synthesis of two meta-modified terpyridyl ligands with selective self-complementary coordination moiety are reported and their capability to assemble into two hourglass-shaped nanocages SA and SB is demonstrated. Within these 3D structures, the meta-modified terpyridyl unit preferably coordinates with itself to serve as concave part. By changing the arm length of the ligands, hexamer (SA) and tetramer (SB) are obtained respectively. In-depth studies on the assembly mechanism of SA and SB indicate that the dimers could be formed first via self-complementary coordination and play crucial roles in controlling the final structures. Moreover, both SA and SB can go through hierarchical self-assemblies in solution as well as on solid-liquid interface, which are characterized by transmission electron microscope (TEM) and scanning tunneling microscopy (STM). It is further demonstrated that various higher-order assembly structures can be achieved by tuning the environmental conditions.

"Lock and Key" and "Induced-Fit" Host-Guest Models in Two Digold(I)-Based Metallotweezers

Two different metallotweezers, each with two pyrene-imidazolylidene-gold(I) arms, were used as hosts for a series of planar aromatic guests. The metallotweezer with a dibenzoacridinebis(alkynyl) spacer (1) orients the two pyrene-imidazolylidene-gold(I) arms in a parallel disposition, with an interpanel distance of about 7 Å. The second metallotweezer (2) contains a carbazolylbis(alkynyl) spacer that directs the two pyrene panels in a diverging orientation. Determination of the association constants via ¹H NMR titrations demonstrates that the binding strength shown by 1 is significantly larger than that found by 2, with binding affinities as large as 10⁴ M^-1 (in CDCl₃), for the encapsulation of N,N'-dimethylnaphthalenetetracarboxydiimide with 1. The differences in the binding affinities are due to binding models associated with formation of the related host-guest complexes. While 1 operates via a "lock and key" model, in which the host does not suffer distortions upon formation of the inclusion complex, 2 operates via a guest-induced fit model. The large association constants shown by 1 with two planar guests were used for promotion of the template-directed synthesis of 1, which in the absence of an external template is produced in an equimolecular mixture with its self-aggregated congener, clippane [1₂]. This observation strongly suggests that the mechanically interlocked clippane is formed through a self-template-directed mechanism, while bonds are broken/formed during the synthetic protocol.

Instantaneous High-Resolution Visual Imaging of Latent Fingerprints in Water Using Color-Tunable AIE Pincer Complexes

Instant visualization of latent fingerprints is developed by using a series of water-soluble terpyridine zinc complexes as aggregation-induced emission probes in pure water, under UV light or ambient sunlight. By simply soaking, or spraying with an aqueous solution of the probe, bright yellow fluorescence images with high contrast and resolution are readily developed on various surfaces including tinfoil, glass, paper, steel, leather, and ceramic tile. Remarkably, latent fingerprints can be visualized within seconds including details of whorl and sweat pores. The color of emission can be tuned from blue to orange by modifying the pincer ligands, allowing direct imaging under sunlight. These inexpensive, water-resistant, and color-tunable probes provide a practical approach for latent fingerprints recording and analysis, security protection, as well as criminal investigation in different scenarios.

The New Di-Gold Metallotweezer Based on an Alkynylpyridine System

We developed a simple method to prepare one gold-based metallotweezer with two planar Au-pyrene-NHC arms bound by a 2,6-bis(3-ethynyl-5-tert-butylphenyl)pyridine unit. This metallotweezer is able to bind a series of polycyclic aromatic hydrocarbons through the π-stacking interactions between the polyaromatic guests and the pyrene moieties of the NHC ligands. The metallotweezer was also used as a host for the encapsulation of planar metal complexes, such as the Au(III) complex [Au(C^N^C)(C≡CC₆H₄-OCH₃-p)], for which there is a large binding constant of 946 M⁻¹.

Design and Self-Assembly of Macrocycles with Metals at the Corners Based on Dissymmetric Terpyridine Ligands

Terpyridine-based discrete supramolecular architectures with metal ions in the corners have rarely been reported. Herein, we report two dissymmetric terpyridyl ligands LA and LB decorated at the 5-position and 4-position of terpyridine respectively. The complexes constructed by the self-assembly of LA and LB with Zn(II) exhibit hand-circle-like structures. Moreover, all Zn(II) are successfully fixed in the corners. A series of dimeric to hexameric macrocycles is obtained by head-to-tail connections with changing concentration. This work will pave the way for preparation of more elaborate self-assembled structures based on dissymetric ligands.

Guest-Driven Self-Assembly and Chiral Induction of Photofunctional Lanthanide Tetrahedral Cages

Chiral luminescent lanthanide-organic cages have many potential applications in enantioselective recognition, sensing, and asymmetric catalysis. However, due to the paucity of structures and their limited cavities, host-guest chemistry with lanthanide-organic cages has remained elusive so far. Herein, we report a guest-driven self-assembly and chiral induction approach for the construction of otherwise inaccessible Ln₄L₄-type (Ln = lanthanide ions, i.e., Eu^III, Tb^III; L = ligand) tetrahedral hosts. Single crystal analyses on a series of host-guest complexes reveal remarkable guest-adaptive cavity breathing on the tetrahedral cages, reflecting the advantage of the variation tolerance on coordination geometry of the f-elements. Meanwhile, noncovalent confinement of pyrene within the lanthanide cage not only leads to diminishment of its excimer emission but also facilitates guest to host energy transfer, opening up a new sensitization window for the lanthanide luminescence on the cage. Moreover, stereoselective self-assembly of either Λ₄- or Δ₄- type Eu₄L₄ cages has been realized via chiral induction with R/S-BINOL or R/S-SPOL templates, as confirmed by NMR, circular dichroism (CD), and circularly polarized luminescence (CPL) with high dissymmetry factors (g_lum) up to ±0.125.

A triple-pore tessellated square array by a metal-hexagonal ligand with reinforced tetra-connectors

Tessellation of nine polygons into a 3 × 3 array is accomplished by the self-assembly of specifically designed hexaruthenium macrocycles containing a tetrapod ligand. Differing from the hexagon-containing bipod ligand, more connections lead to a giant discrete stable higher-order assembly. The formed tessellated square array possesses three different kinds of pores and each pore contains different metal ions, including one central tetragonum (Zn₄), four corner hexagons (Ru₆), and four side irregular hexagons (Ru₂Zn₂), which provides a promising way to fabricate multichannel architectures.

Strong π-stacking causes unusually large anisotropic thermal expansion and thermochromism

π-stacking in ground-state dimers/trimers/tetramers of N-butoxyphenyl(naphthalene)diimide (BNDI) exceeds 50 kcal ⋅ mol^-1 in strength, drastically surpassing that for the ^*3[pyrene]₂ excimer (∼30 kcal ⋅ mol^-1; formal bond order = 1) and similar to other weak-to-moderate classical covalent bonds. Cooperative π-stacking in triclinic (BNDI-T) and monoclinic (BNDI-M) polymorphs effects unusually large linear thermal expansion coefficients (α _a , α _b , α _c , β) of (452, -16.8, -154, 273) × 10^-6 ⋅ K^-1 and (70.1, -44.7, 163, 177) × 10^-6 ⋅ K^-1, respectively. BNDI-T exhibits highly reversible thermochromism over a 300-K range, manifest by color changes from orange (ambient temperature) toward red (cryogenic temperatures) or yellow (375 K), with repeated thermal cycling sustained for over at least 2 y.

Perylene Bisimide-Cored Supramolecular Coordination Complexes: Interplay between Ensembles, Excited State Processes, and Aggregation Behaviors

Manipulating the optical properties of fluorescent species is challenging owing to complicated and tedious synthetic works. Herein, the photophysical properties of perylene bisimide (PBI) were effectively tuned by varying the geometrical arrangement of PBI moieties within supramolecular coordination complexes (SCCs), where a PBI-based dicycle (2) and a trigonal prism (3) were generated via using a typical 90° Pt(II) reagent, cis-(PEt₃ )₂ Pt(OTf)₂ -based coordination-driven self-assembly approach. The ligand, an ortho-tetrapyridiyl-PBI (1), exhibits a moderate fluorescence quantum yield (∼13 %) and efficient inter-system crossing (ISC). 2, however, is much more emissive with a fluorescence quantum yield of ∼41 %, and the relevant ISC process is significantly hindered. The fluorescence quantum yield of 3 is merely ∼6 % due to the observed symmetry-breaking charge separation (SB-CS), which turns to triplet state upon charge recombination. Interestingly, 3 could be fully transformed into 2 by simply adding a suitable amount of a 90° Pt(II)-based neutral triangle. Moreover, 2 tends to form discrete dimers both in crystal and solution states, but 3 does not show the property. Therefore, controlling geometrical arrangement of fluorophores through coordination-driven self-assembly could be taken as another effective way to tune their excited state relaxation pathways and construct high-performance optical molecular materials, which generally have to be prepared via organic synthesis.

Clippane: A Mechanically Interlocked Molecule (MIM) Based on Molecular Tweezers

In this study we report the preparation of a new mechanically interlocked molecule formed by the self-aggregation of two metallotweezers composed by two pyrene-imidazolylidene gold(I) arms and a pyridine-centered pentacyclic bis-alkynyl linker. The mechanically interlocked nature of this molecule arises from the presence of the bulky tert-butyl groups attached to the sides of the pyrene moieties of the arms of the tweezer, which act as stoppers avoiding the dissociation of the self-aggregated metallotweezer dimer once it is formed. By combining experimental techniques, we were able to confirm the mechanically interlocked nature of this molecule in solution, in the gas phase and in the solid state. The behavior of the tert-butyl substituted tweezer differs greatly form that shown by the tweezer lacking of these bulky groups, whose dimeric structure is in equilibrium with the monomeric structure, therefore not showing any mechanical coercion that avoids the disassembly of the self-aggregated structure.

Artificial Light-Harvesting Systems Based on AIEgen-branched Rotaxane Dendrimers for Efficient Photocatalysis

Aiming at the construction of novel platform for efficient light harvesting, the precise synthesis of a new family of AIEgen-branched rotaxane dendrimers was successful realized from an AIEgen-functionalized [2]rotaxane through a controllable divergent approach. In the resultant AIE macromolecules, up to twenty-one AIEgens located at the tails of each branches, thus making them the first successful example of AIEgen-branched dendrimers. Attributed to the solvent-induced switching feature of the rotaxane branches, the integrated rotaxane dendrimers displayed interesting dynamic feature upon the aggregation-induced emission (AIE) process. Moreover, novel artificial light-harvesting systems were further constructed based on these AIEgen-branched rotaxane dendrimers, which revealed impressive generation-dependent photocatalytic performances for both photooxidation reaction and aerobic cross-dehydrogenative coupling (CDC) reaction.

Fine-Tuning Metal and Ligand-Centered Redox Potentials of Homoleptic Bis-Terpyridine Complexes with 4'-Aryl Substituents

Homoleptic transition-metal complexes of 2,2':6',2″-terpyridine (terpy) and substituted derivatives of the form [M(R-terpy)₂]²⁺ display a wide range of redox potentials that correlate well to the Hammett parameter of the terpy substituents. Less is known about the impact of incorporating a phenyl spacer between the functional group responsible for controlling the electron density of terpy and how that translates to metal complexes of the form [M(4'-aryl-terpy)₂]²⁺, where M = Mn, Fe, Co, Ni, and Zn. Herein, we report our studies on these complexes revealed a good correlation of redox potentials of both metal- and ligand-centered events with the Hammett parameters of the aryl substituents, regardless of aryl-substitution pattern (i.e., the presence of multiple functional groups, combinations of withdrawing and donating functional groups). The phenyl spacer results in 60-80% attenuation of electron density as compared to the 4'-substituted terpy analogue, depending on the metal and redox couple analyzed. Density functional theory calculations performed on a simple model system revealed a strong correlation between the Hammett parameters and lowest unoccupied molecular orbital energies of the corresponding substituted pyridine models, thus serving as an inexpensive predictive tool when coupled with electrochemical data. Overall, these data suggest that such ligand modifications may be used in combination with previous approaches to further fine-tune the redox potentials of homoleptic transition-metal complexes, which may have applications in photochemical and electrochemical catalytic processes.

Shape-Adaptability and Redox-Switching Properties of a Di-Gold Metallotweezer

The use of a carbazolyl‐connected di‐gold(I) metallotweezer for the encapsulation of several electron‐poor organic substrates, and a planar Au(III) complex containing a CNC pincer ligand, is described. The binding affinity of the receptor depends on the electron‐deficient character of the planar guest, with larger association constants found for the more electron‐poor guests. The X‐ray diffraction molecular structures of two host:guest adducts show that the host approaches its arms in order to facilitate the optimum interaction with the surface of the planar guests, in a clear example of an guest‐induced fit conformational arrangement. The electrochemical studies of the encapsulation of N,N’‐dimethyl‐naphthalenetetracarboxy diimide (NTCDI) show that the redox active guest is released from the receptor upon one electron reduction, thus constituting an example of redox‐switchable binding.

Induction, control, and rationalization of supramolecular chirogenesis using metalloporphyrin tweezers: a structure-function correlation

Supramolecular chirogenesis is one of the most rudimentary topics in the interdisciplinary sciences and essentially deals with various natural processes and innovative modern technologies. A comprehensive and rigorous understanding of such phenomenon is necessary to have a clear insight into the fundamental mechanisms and the various controlling factors, which would eventually lead to a range of practical applications of chiral supramolecular science. Metalloporphyrin tweezers have been extensively employed for such chirogenic processes due to their exciting physicochemical and tunable spectral properties, large stabilities, easily available synthetic protocols, and excellent abilities to form molecular assemblies. During the last few decades, various metalloporphyrin tweezers have been developed and considerably utilized by several research groups for assigning the absolute configuration to a variety of chiral diamines, conjugates of primary and secondary amines, amino alcohols, secondary alcohols, α-chiral carboxylic acids, etc. Our group has been at the forefront in trying to establish the structure-property correlation in this important area of interdisciplinary research. A brief account of our systematic investigation for understanding the underpinning mechanism of chirality induction and control at the molecular level over the last few years is presented in this Perspective article. The comprehensive understanding of such mechanistic details will be helpful in understanding various natural processes and designing modern technologies for various chirogenic functions in the fields of molecular sensors, nanotechnology, and supramolecular chemistry.

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.

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 .

Emissive Supramolecular Systems Based on Reversible Bond Formation and Noncovalent Interactions

Noncovalent interactions and reversible bond formations are widely seen in natural systems for the construction of sophisticated molecular systems that perform various biological processes. Inspired by the natural systems, luminescent supramolecular systems constructed by coordination-driven self-assembly and homometallic metal-metal interations have been studied increasingly. These supramolecular systems show fascinating luminescent behaviors that are not observed from single components. This review summarizes our progress in the development of two types of unique luminescent supramolecular systems. The mononuclear Pt(II) complex units can sandwich coinage metal ions to form heteropolynuclear complexes involving heterometallic metal-metal interactions. A close proximity of the two or three different metal ions by the noncovalent forces lead to orbital overlapping among the coinage metal ions and the Pt(II) complex units, showing emission color change accompanied with structural transformation and reversible metal binding behaviors. Emissive host-guest systems consisting of mononuclear metal complexes and a hydrogen-bonded capsule are also developed, that show a unique encapsulation-induced emission enhancement (EIEE) behavior.

Stimuli-Responsive Two-Dimensional Supramolecular Polymers Based on Trinuclear Platinum(II) Scaffolds: Reversible Modulation of Photoluminescence, Cavity Size, and Water Permeability

Here we report the first two-dimensional (2D) supramolecular polymer, which has varying structure and function arising from the perturbation of noncovalent metal···metal interactions in response to acid-base stimuli. This 2D assembly possesses a positively charged, honeycomb-like nanostructure consisting of trinuclear alkynylplatinum(II) terpyridine complexes appended with acid-sensitive dimethylamino groups. Upon addition of acids and bases, reversible switching mediated by protonation and deprotonation of dimethylamino and dimethylammonium moieties intrinsically alters the positive charge density of the constituent cationic units, which causes interior cavities to adaptively adjust their size, accompanied by drastic photoluminescence changes. When water molecules pass through the membranes obtained from 2D supramolecular polymers, the permeating flux can also be tuned by the pH values of the buffer media. This work paves the way toward supramolecularly engineered 2D smart materials with stimuli-responsive properties.

N-Heterocyclic Carbenes: A Door Open to Supramolecular Organometallic Chemistry

The field of metallosupramolecular chemistry is clearly dominated by the use of O-, N-, and P-donor Werner-type polydentate ligands. These molecular architectures are of high interest because of their wide range of applications, which include molecular encapsulation, stabilization of reactive species, supramolecular catalysis, and drug delivery, among others. Only recently, organometallic ligands have allowed the preparation of a variety of supramolecular coordination complexes, and the term supramolecular organometallic complexes (SOCs) is gaining space within the field of metallosupramolecular chemistry. While the early examples of SOCs referred to supramolecular architectures mostly containing bisalkenyl, diphenyl, or bisalkynyl linkers, the development of SOCs during the past decade has been boosted by the parallel development of multidentate N-heterocyclic carbene (NHC) ligands. The first examples of NHC-based SOCs referred to supramolecular assemblies based on polydentate NHC ligands bound to group 11 metals. However, during the last 10 years, several planar poly-NHC ligands containing extended π-conjugated systems have facilitated the formation of a large variety of architectures in which the supramolecular assemblies can contain metals other than Cu, Ag, and Au. Such ligands are Janus di-NHCs and trigonal-planar tris-NHCs-most of them prepared by our research group-which have allowed the preparation of a vast range of NHC-based metallosupramolecular compounds with interesting host-guest chemistry properties. Although the number of SOCs has increased in the past few years, their use for host-guest chemistry purposes is still in its earliest infancy. In this Account, we describe the achievements that we have made during the last 4 years toward broadening the applications of planar extended π-conjugated NHC ligands for the preparation of organometallic-based supramolecular structures, including their use as hosts for some selected organic and inorganic guests, together with the catalytic properties displayed by some selected host-guest inclusion complexes. Our contribution describes the design of several Ni-, Pd-, and Au-based metallorectangles and metalloprisms, which we used for the encapsulation of several organic substrates, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes. The large binding affinities found are ascribed to the incorporation of two cofacial panels with large π-conjugated systems, which provide the optimum conditions for guest recognition by π-π-stacking interactions. We also describe a series of digold(I) metallotweezers for the recognition of organic and inorganic substrates. These metallotweezers were used for the recognition of "naked" metal cations and polycyclic aromatic hydrocarbons. The recognition properties of these metallotweezers are highly dependent on the nature of the rigid connector and of the ancillary ligands that constitute the arms of the tweezer. A peculiar balance between the self-aggregation properties of the tweezer and its ability to encapsulate organic guests is observed.

Daisy Chain Dendrimers: Integrated Mechanically Interlocked Molecules with Stimuli-Induced Dimension Modulation Feature

The precise construction of the high-order mechanically interlocked molecules (MIMs) with well-defined topological arrangements of multiple mechanically interlocked units has been a great challenge. Herein, we present the first successful preparation of a new family of daisy chain dendrimers, in which the individual [c2]daisy chain rotaxane units serve as the branches of dendrimer skeleton. In particular, the third-generation daisy chain dendrimer with 21 [c2]daisy chain rotaxane moieties was realized, which might be among the most complicated discrete high-order MIMs comprised of multiple [c2]daisy chain rotaxane units. Interestingly, such unique topological arrangements of multiple stimuli-responsive [c2]daisy chain rotaxanes endowed the resultant daisy chain dendrimers controllable and reversible nanoscale dimension modulation through the collective and amplified extension/contraction of each [c2]daisy chain rotaxane branch upon the addition of acetate anions or DMSO molecules as external stimulus. Furthermore, on the basis of such an intriguing size switching feature of daisy chain dendrimers, dynamic composite polymer films were constructed through the incorporation of daisy chain dendrimers into polymer films, which could undergo fast, reversible, and controllable shape transformations when DMSO molecules were employed as stimulus. The successful merging of [c2]daisy chain rotaxanes and dendrimers described herein provides not only a brand-new type of high-order mechanically interlocked systems with well-defined topological arrangements of [c2]daisy chain rotaxanes, but also a successful and practical approach toward the construction of supramolecular dynamic materials.

Chiral auxiliary and induced chiroptical sensing with 5d/4f lanthanide-organic macrocycles

A stereocontrolled self-assembly of 5d/4f heterometal-organic macrocycles has been realized by a post-assembly chiral auxiliary approach. Interligand π-π stacking interactions promoted chiral transfer from point-chirality of the auxiliary ligand to metal centers and then the helicates to produced CD and CPL active europium assemblies, facilitating a feasible enantiomeric excess determination by induced chiroptical signals.

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.

Expanding the Toolbox for Label-Free Enzyme Assays: A Dinuclear Platinum(II) Complex/DNA Ensemble with Switchable Near-IR Emission

Switchable luminescent bioprobes whose emission can be turned on as a function of specific enzymatic activity are emerging as important tools in chemical biology. We report a promising platform for the development of label-free and continuous enzymatic assays in high-throughput mode based on the reversible solvent-induced self-assembly of a neutral dinuclear Pt(II) complex. To demonstrate the utility of this strategy, the switchable luminescence of a dinuclear Pt(II) complex was utilized in developing an experimentally simple, fast (10 min), low cost, and label-free turn-on luminescence assay for the endonuclease enzyme DNAse I. The complex displays a near-IR (NIR) aggregation-induced emission at 785 nm in aqueous solution that is completely quenched upon binding to G-quadruplex DNA from the human c-myc oncogene. Luminescence is restored upon DNA degradation elicited by exposure to DNAse I. Correlation between near-IR luminescence intensity and DNAse I concentration in human serum samples allows for fast and label-free detection of DNAse I down to 0.002 U/mL. The Pt(II) complex/DNA assembly is also effective for identification of DNAse I inhibitors, and assays can be performed in multiwell plates compatible with high-throughput screening. The combination of sensitivity, speed, convenience, and cost render this method superior to all other reported luminescence-based DNAse I assays. The versatile response of the Pt(II) complex to DNA structures promises broad potential applications in developing real-time and label-free assays for other nucleases as well as enzymes that regulate DNA topology.

Construction of Type III-C Rotaxane-Branched Dendrimers and Their Anion-Induced Dimension Modulation Feature

Starting from a novel rotaxane building block with dendrimer growth sites being located at both the wheel and axle component, we realized the successful construction of a new family of rotaxane-branched dendrimers, i.e., Type III-C rotaxane-branched dendrimers, up to fourth generation as a highly branched [46]rotaxane through a controllable divergent approach. In the resultant rotaxane-branched dendrimers, the wheel components of the rotaxane units are located on the branches as well as at the branching points, making them excellent candidates to mimic the amplified collective molecular motions. Thus, taking advantage of the urea moiety inserted into the axle components of the rotaxane units as the binding sites, the addition or removal of acetate anion as stimulus endows the individual rotaxane unit a switchable feature that lead to a collective expansion-contraction motion of the integrated rotaxane-branched dendrimers, thus allowing for the remarkable and reversible size modulation. Such a three-dimensional size switching feature makes Type III-C rotaxane-branched dendrimers a very promising platform toward the fabrication of novel dynamic smart materials.