Self-assembled coordination complexes from various palladium(II) components and bidentate or polydentate ligands

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Tunable Fullerene Affinity of Cages, Bowls and Rings Assembled by PdII Coordination Sphere Engineering

For metal-mediated host compounds, the development of strategies to reduce symmetry and introduce multiple functionalities in a non-statistical way is a challenging task. We show that the introduction of steric stress around the coordination environment of square-planar PdII cations and bis-monodentate nitrogen donor ligands allows to control the size and shape of the assembled product, from [Pd2 L4 ] cages over [Pd2 L3 ] bowl-shaped structures to [Pd2 L2 ] rings. Therefore, banana-shaped ligand backbones were equipped with pyridines, two different quinoline isomers and acridine, the latter three introducing steric congestion through hydrogen substituents on annelated benzene rings. Differing behavior of the four resulting hosts towards the binding of C60 and C70 fullerenes was studied and related to structural differences by NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. The three cages based on pyridine, 6-quinoline or 3-quinoline donors were found to either bind C60 , C70 or no fullerene at all.

[2.2]Paracyclophane bis(pyridine)-based metallosupramolecular rhombs in the gas phase: Competitive cleavage of non-covalent and weak covalent bonds

The gas-phase fragmentation behavior of self-assembled metallo-supramolecular rhombs based on an unusual chiral [2.2]paracyclophane bis(pyridine) ligand is studied by collision-induced dissociation mass spectrometry. The fragmentation patterns strongly depend on the charge state of the respective mass-selected aggregate. For the doubly charged ions, simple symmetric fragmentation is observed in full accordance with previous results reported for related metallo-supramolecular species. The triply charged species cleaves unsymmetrically which can be rationalized by a preferred formation of ions with low charge density. CID of the quadruply charged rhomb reveals a complex fragmentation. Besides ligand oxidation to the radical cation, facile cleavage of the central covalently bound part of the [2.2]paracyclophane ligand takes place which is even preferred over rupture of the weak dative pyridine-Pd bond.

Bent Anthracene Dimers as Versatile Building Blocks for Supramolecular Capsules

This Account provides a comprehensive summary of our 1-decade-long investigations into bent anthracene dimers as versatile building blocks for supramolecular capsules. The investigations initiated in 2008 with the design of an anthracene dimer with a meta-phenylene spacer bearing two substituents on the convex side. Using the bent polyaromatic building block, we began to develop novel supramolecular capsules from two different synthetic approaches. One is a coordination approach, which was pursued by converting the building block into a bent ligand with two pyridine units at the terminal positions. The ligands quantitatively assemble into an M₂L₄-type capsule through coordination bonding with metal ions. The other is a π-stacking approach, which was followed by utilizing the block as a bent amphiphilic molecule with two trimethylammonium groups at the spacer. In water, the amphiphiles spontaneously assemble into a micelle-type capsule through the hydrophobic effect and π-stacking interactions. Simple modification of the building block allowed us to prepare a wide variety of coordination capsules as well as π-stacking capsules, bearing different hydrophilic side-chains, terminal substitutions, connecting units, polyaromatic panels, or spacer units. The coordination capsule possesses a rigid cavity, with a diameter of ∼1 nm, surrounded by multiple anthracene panels. The spherical polyaromatic cavity binds various synthetic molecules (e.g., paracyclophanes, corannulene, BODIPY, and fullerene C₆₀) in aqueous solutions. With the aid of the polyaromatic shell, photochemically and thermally reactive radical initiators and oligosulfurs are greatly stabilized in the cavity. Biomolecules such as hydrophilic sucrose and oligo(lactic acid)s as well as hydrophobic androgenic hormones are bound by the capsule with high selectivity. In addition, long amphiphilic poly(ethylene oxide)s are threaded into the closed shell of the capsule(s) to generate unusual pseudorotaxane-shaped host-guest complexes in water. In contrast, the π-stacking capsule furnishes a flexible cavity, adaptable to the size and shape of guest molecules, encircled by multiple anthracene panels. In water, the capsule binds hydrophobic fluorescent dyes (e.g., Nile red and DCM) in the cavity. Simple grinding of the bent amphiphile with highly hydrophobic nanocarbons such as fullerenes, nanographenes, and carbon nanotubes (followed by sonication) as well as metal-complexes such as Cu(II)-phthalocyanines and Mn(III)-tetraphenylporphyrins leads to the efficient formation of water-soluble host-guest complexes upon encapsulation. Red emission from otherwise water-deactivated Eu(III)-complexes is largely enhanced in water through encapsulation. Moreover, the incorporation of pH- and photoswitches into the amphiphile affords stimuli-responsive π-stacking capsules, capable of releasing bound guests by the addition of acid and light irradiation, respectively, in water. The host functions of the coordination and π-stacking capsules are complementary to each other, which enables selection of the capsule-type depending on the envisioned target. We are convinced that continued investigation of the present supramolecular capsules featuring the bent anthracene dimer and its derivatives will further increase their value as advanced molecular tools for synthetic, analytical, material, biological, and/or medical applications.

Metal and Organic Templates Together Control the Size of Covalent Macrocycles and Cages

Covalent macrocycles and three-dimensional cages were prepared by the self-assembly of di- or tritopic anilines and 2,6-diformylpyridine subcomponents around palladium(II) templates. The resulting 2,6-bis(imino)pyridyl-Pd^II motif contains a tridentate ligand, leaving a free coordination site on the Pd^II centers, which points inward. The binding of ligands to the free coordination sites in these assemblies was found to alter the product stability, and multitopic ligands could be used to control product size. Multitopic ligands also bridged metallomacrocycles to form higher-order supramolecular assemblies, which were characterized via NMR spectroscopy, mass spectrometry, and X-ray crystallography. An efficient method was developed to reduce the imine bonds to secondary amines, leading to fully organic covalent macrocycles and cages that were inaccessible through other means.

Palladium(II)-Based Self-Assembled Heteroleptic Coordination Architectures: A Growing Family

Metal-driven self-assembly is one of the most effective approaches to lucidly design a large range of discrete 2D and 3D coordination architectures/complexes. Palladium(II)-based self-assembled coordination architectures are usually prepared by using suitable metal components, in either a partially protected form (PdL') or typical form (Pd; charges are not shown), and designed ligand components. The self-assembled molecules prepared by using a metal component and only one type of bi- or polydentate ligand (L) can be classified in the homoleptic series of complexes. On the other hand, the less explored heteroleptic series of complexes are obtained by using a metal component and at least two different types of non-chelating bi- or polydentate ligands (such as L^a and L^b ). Methods that allow the controlled generation of single, discrete heteroleptic complexes are less understood. A survey of palladium(II)-based self-assembled coordination cages that are heteroleptic has been made. This review article illustrates a systematic collection of such architectures and credible justification of their formation, along with reported functional aspects of the complexes. The collected heteroleptic assemblies are classified here into three sections: 1) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal; 2) [(PdL')_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is different; and 3) [Pd_m (L^a )_x (L^b )_y ]-type complexes, in which the denticity of L^a and L^b is equal. Representative examples of some important homoleptic architectures are also provided, wherever possible, to set a background for a better understanding of the related heteroleptic versions. The purpose of this review is to pave the way for the construction of several unique heteroleptic coordination assemblies that might exhibit emergent supramolecular functions.

Design of a double-decker coordination cage revisited to make new cages and exemplify ligand isomerism

The complexation study of cis-protected and bare palladium(II) components with a new tridentate ligand, i.e., pyridine-3,5-diylbis(methylene) dinicotinate (L1) is the focus of this work. Complexation of cis-Pd(tmeda)(NO₃)₂ with L1 at a 1:1 or 3:2 ratio produced [Pd(tmeda)(L1)](NO₃)₂ (1a). The reaction mixture obtained at 3:2 ratio upon prolonged heating, produced a small amount of [Pd₃(tmeda)₃(L1)₂](NO₃)₆ (2a). Complexation of Pd(NO₃)₂ with L1 at a 1:2 or 3:4 ratios afforded [Pd(L1)₂](NO₃)₂ (3a) and [(NO₃)₂@Pd₃(L1)₄](NO₃)₄ (4a), respectively. The encapsulated NO₃^– ions of 4a undergo anion exchange with halides (F^–, Cl^– and Br^– but not with I^–) to form [(X)₂@Pd₃(L1)₄](NO₃)₄5a–7a. The coordination behaviour of ligand L1 and some dynamic properties of these complexes are compared with a set of known complexes prepared using the regioisomeric ligand bis(pyridin-3-ylmethyl)pyridine-3,5-dicarboxylate (L2). Importantly, a ligand isomerism phenomenon is claimed by considering complexes prepared from L1 and L2.

Open versus Closed Polyaromatic Nanocavity: Enhanced Host Abilities toward Large Dyes and Pigments

Host functions of polyaromatic nanocavities were revealed by using an M₂ L₄ molecular cage and capsule. On the basis of the previously reported M₂ L₄ capsule with a closed polyaromatic cavity, a new M₂ L₄ cage (as a mixture of the isomers) was prepared by the quantitative assembly of two metal ions and four desymmetrized bispyridine ligands with a single polyaromatic panel. The obtained, open nanocavity of the cage exhibited enhanced binding abilities toward large dyes and pigments in water. For example, two molecules of coumarin dyes were bound in the nanocavity and showed strong whitish emission (up to Φ_F =34 %). Furthermore, metallopigments, the sizes of which are larger than the inner cavities of the cage and capsule, were bound only in the open polyaromatic nanocavity of the cage to give water-soluble 1:1 host-guest complexes.

Dynamic Molecular Metamorphism Involving Palladium-Assisted Dimerization of π-Cation Radicals

A dynamic supramolecular approach is developed to promote the π-dimerization of viologen radicals at room temperature and in standard concentration ranges. The approach involves cis- or trans-protected palladium centers serving as inorganic hinges linking two functionalized viologens endowed with metal-ion coordinating properties. Based on detailed spectroscopic, electrochemical and computational data, we show that the one-electron electrochemical reduction of the viologen units in different dynamic metal/ligand mixtures leads to the formation of the same intramolecular π-dimer, regardless of the initial environment around the metallic precursor and of the relative ratio between metal and ligand initially introduced in solution. The large-scale electron-triggered reorganization of the building blocks introduced in solution thus involves drastic changes in the stoichiometry and stereochemistry of the palladium/viologen complexes proceeding in some cases through a palladium centered trans→cis isomerization of the coordinated ligands.

Strongly luminescent 5d/4f heterometal–organic macrocycles with open metal sites: post-assembly modification and sensing

Strongly luminescent 5d/4f heterometal–organic macrocycles featuring open metal sites have been constructed, along with their post-assembly modification and sensing properties.

Chiral self-sorting behaviour of [2.2]paracyclophane-based bis(pyridine) ligands

[2.2]Paracyclophane-based bis(pyridine) ligands form dinuclear complexes upon coordination to palladium(ii) ions, however, with distinct differences concerning their chiral self-sorting ability.

A mono-metallic Pd(ii)-cage featuring two different polar binding sites

A tetra-α super aryl-extended tetra-pyridyl calix[4]pyrrole self-assembles into a mono-Pd(ii) cage featuring two different and converging polar binding sites.

Discrete and Polymeric Self-Assembled Palladium(II) Complexes as Supramolecular Gelators

Supramolecular gels prepared from low-molecular-weight gelators have been extensively explored. However, the exploitation of discrete or polymeric metal complexes as gelators is a relatively recent trend. The synthesis of self-assembled coordination complexes from palladium(II) and selected ligands is well established, but the potential of these complexes as gelators is a less explored treasure. Herein we focus on the gelation abilities of some self-assembled palladium(II) complexes and the resulting unique properties. First, discrete complexes with PdL, PdL₂ , Pd₂ L, Pd₂ L₂ , Pd₂ L₄ , and Pd₃ L₆ compositions are discussed. Second, gelation behavior promoted by coordination-polymer-like gelators formed in situ is explored. These gel samples have been employed in catalysis and the uptake of organic and dye molecules from the solution and gas phases. It is concluded that untapped unique properties can be realized by further exploration of designer palladium(II) complexes.

Mixed-Ligand Metal-Organic Frameworks and Heteroleptic Coordination Cages as Multifunctional Scaffolds-A Comparison

Porous nanostructures and materials based on metal-mediated self-assembly have developed into a vibrantly studied subdiscipline of supramolecular chemistry during the past decades. In principle, two branches of such coordination compounds can be distinguished: Metal-organic frameworks (MOFs) on the one side represent infinite porous networks of metals or metal clusters that are connected via organic ligands to give solid-state materials. On the other hand, metal-organic cages (MOCs) are discrete and soluble systems with only a limited number of pores. Formation of a particular structure type is achieved by carefully balancing the donor site angles within the ligands as well as the nature and coordination geometry of the metal component. Years of research on MOFs and MOCs has yielded numerous types of well-defined porous crystals and complex supramolecular architectures. Since various synthetic routes and postsynthetic modification methods have been established, the focus of recent developments has moved toward the preparation of multifunctional systems that are able to mimic the structural and functional complexity of natural enzymes. This Account compares different strategies to prepare multifunctional MOFs and heteroleptic MOCs and gives a perspective on where to move forward. While the preparative toolbox for multifunctional MOFs is already quite mature, pore accessibility and substrate diffusion within the crystal have been identified as major challenges yet to be overcome. Only recently have a set of different strategies for the assembly of heteroleptic MOCs been developed. Such multifunctional cages can be formed from either partially protected or "naked" metal cations. Controlled assembly, producing single products rather than statistical mixtures, leans on assembly-dependent approaches making use of either steric effects or shape complementarity between the ligands. Further strategies include coordination-site engineering and hierarchical assembly of preformed components. The main challenge with heteroleptic, functional MOCs is to find a balance between the required dynamic assembly fidelity and the stability of the resulting system under operating conditions. If these limitations can be overcome in the future, chemists will be able to design multifunctional systems of similar activity and complexity as nature's enzymes from simple and easily accessible synthetic building blocks. Major impacts on chemical sensing, small-molecule recognition and sequestration, drug delivery, and catalysis will be achieved by these materials.

Ligand Isomerism in Coordination Cages

Complexation reactions of palladium(II) nitrate with a set of 3-pyridyl appended nonchelating bidentate ligands possessing regioisomeric phenylene-diurea functionalities as spacers were carried out. The ligands utilized in this study are 1,1'-(1,2-phenylene)bis(3-(pyridin-3-yl)urea), L1; 1,1'-(1,3-phenylene)bis(3-(pyridin-3-yl)urea), L2; and 1,1'-(1,4-phenylene)bis(3-(pyridin-3-yl)urea), L3. The complexation reactions of the ligands (L1, L2, and L3) with palladium(II) produced single discrete isomeric cages (1, 2, and 3) of Pd₂L₄ formulation in each case and thereby illustrated ligand-isomerism in coordination cages. All 16 hydrogen atoms of eight urea moieties present in four ligand strands are delineated completely endohedrally in cage 1 and completely exohedrally in cage 3, whereas cage 2 exhibited half of the urea hydrogens in exohedral locations and the remaining half in endohedral locations. In addition to the variable number of solvent molecules, the cavities of cages 1 and 2 lodged four and two nitrate ions, respectively, using the endohedral (H)_urea atoms (i.e., NH groups) as binding sites, whereas the cavity of 3 remained anion free. The abilities of the complexes 1-3 for adsorption of CO₂ gas are demonstrated, and their behaviors are compared.

A Rotaxane-like Cage-in-Ring Structural Motif for a Metallosupramolecular Pd6 L12 Aggregate

A BODIPY-based bis(3-pyridyl) ligand undergoes self-assembly upon coordination to tetravalent palladium(II) cations to form a Pd₆ L₁₂ metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane-like cage-in-ring arrangement. In this assembly the ligand adopts two different conformations-a C-shaped one to form a Pd₂ L₄ cage which is located in the center of a Pd₄ L₈ ring consisting of ligands in a W-shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π-stacking between the peripheral BODIPY chromophores and the ligands' skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co-arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane-like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X-ray crystallography.

Self-Assembled PdII6 Molecular Spheroids and Their Proton Conduction Property

A series of molecular spheroids (SP1-SP4) was synthesized using pseudolinear bisimidazole and bisbenzimidazole donors in combination with Pd(NO₃)₂ acceptor via coordination-driven self-assembly. They were characterized by NMR and mass spectrometry, and the solid-state structures of SP1 and SP3 were confirmed by X-ray diffraction. Crystal packing revealed the presence of molecular channels with water molecules in the channels as proton source. All the systems showed proton conductivity across a wide range of temperature and relative humidity. Furthermore, the mode of proton conduction in these molecular spheroids was explored by performing a control experiment using 2,4-dinitrophenol molecule, which indicates that the proton conductivity in the present case increases with increasing surface area of these molecular spheroids.

Multiple Pathways in the Self-Assembly Process of a Pd4L8 Coordination Tetrahedron

The self-assembly of a Pd₄1₈ coordination tetrahedron (Tet) from a ditopic ligand, 1, and palladium(II) ions, [PdPy*₄]²⁺ (Py* = 3-chloropyridine), was investigated by a ¹H NMR-based quantitative approach (quantitative analysis of self-assembly process, QASAP), which allows one to monitor the average composition of the intermediates not observed by NMR spectroscopy. The self-assembly of Tet takes place mainly through three pathways and about half of the Tet structures were produced through the reaction of a kinetically produced Pd₃L₆ double-walled triangle (DWT) and 200-nm-sized large intermediates (Int^L). In two of the three pathways, the leaving ligand (Py*), which is not a component of Tet, catalytically assisted the self-assembly. Such a multiplicity of the self-assembly process of Tet suggests that molecular self-assembly takes place on an energy landscape like a protein-folding funnel.

Metallosupramolecular Architectures Formed with Ferrocene-Linked Bis-Bidentate Ligands: Synthesis, Structures, and Electrochemical Studies

The self-assembly of ligands of different geometries with metal ions gives rise to metallosupramolecular architectures of differing structural types. The rotational flexibility of ferrocene allows for conformational diversity, and, as such, self-assembly processes with 1,1'-disubstituted ferrocene ligands could lead to a variety of interesting architectures. Herein, we report a small family of three bis-bidentate 1,1'-disubstituted ferrocene ligands, functionalized with either 2,2'-bipyridine or 2-pyridyl-1,2,3-triazole chelating units. The self-assembly of these ligands with the (usually) four-coordinate, diamagnetic metal ions Cu(I), Ag(I), and Pd(II) was examined using a range of techniques including ¹H and DOSY NMR spectroscopies, high-resolution electrospray ionization mass spectrometry, X-ray crystallography, and density functional theory calculations. Additionally, the electrochemical properties of these redox-active metallosupramolecular assemblies were examined using cyclic voltammetry and differential pulse voltammetry. The copper(I) complexes of the 1,1'-disubstituted ferrocene ligands were found to be coordination polymers, while the silver(I) and palladium(II) complexes formed discrete [1 + 1] or [2 + 2] metallomacrocyclic architectures.

Coordination-driven self-assembly of chiral palladium(ii)-based supramolecular triangle structures

Chiral structures of palladium(ii)-based triangular supramolecular complexes, with achiral corners and edges, have been characterized in solution.

Controlling orthogonal self-assembly through cis–trans isomerization of a non-covalent palladium complex dimer

Self-assembly of a chiral complex dimer held by 16 hydrogen bonds was controlled through configurational isomerization of the metal center.

Terpyridine-based metallosupramolecular constructs: tailored monomers to precise 2D-motifs and 3D-metallocages

Comprehensive summary of the recent developments in the growing field of terpyridine-based, discrete metallosupramolecular architectures.

Exact mass analysis of sulfur clusters upon encapsulation by a polyaromatic capsular matrix

Structural determination of inorganic clusters relies heavily on mass spectrometry because of, in most cases, their poor responsivities toward nuclear magnetic resonance, ultraviolet/visible, and infrared analyses. Nevertheless, mass spectrometry analysis of oligosulfurs (S n ), which are unique clusters with copious allotropic forms, usually displays their fragment peaks. Here we report that a polyaromatic capsule acts as a supramolecular matrix for the mass determination of the neutral sulfur clusters. Upon encapsulation, molecular ion peaks derived from the host-guest complexes including cyclic S6 and S8 clusters are exclusively detected by common electrospray ionization time-of-flight mass spectrometry analysis. Furthermore, mass spectrometry analysis of a cyclic S12 cluster, which is in situ prepared from two S6 clusters within the matrix upon light irradiation, is achieved by the same way. The present matrix can remarkably stabilize the otherwise labile S6 and S12 clusters in the polyaromatic shell not only under mass spectrometry conditions but also in an ambient solution state.The structures of inorganic clusters are commonly characterized by mass spectrometry (MS), but neutral sulfur clusters heavily fragment under MS conditions, preventing their exact mass determination. Here, the authors successfully perform MS on labile cyclic sulfur clusters by stabilizing them within ionic supramolecular capsules.

Multicavity Metallosupramolecular Architectures

Discrete metallosupramolecular systems are often macrocyclic or cage-like architectures with an accessible internal cavity. Guest molecules can reside within these cavities and much of the interest in these systems is derived from these fascinating host-guest interactions. A range of potential applications stem from the ability of these metallosupramolecular architectures to encapsulate guests. These applications include catalysis or acting as molecular reaction flasks, the molecular scavenging of pollutants, storage of reactive species, and drug delivery. Multicavity metallosupramolecular architectures combine the ability of large hollow assemblies to bind multiple guests concurrently with the binding specificity associated with small cages. A variety of different approaches to generating separate compartments within a single metallosupramolecular assembly have emerged. These include interpenetrated cages, cages with polytopic ligands that have a long backbone, and molecules that have two or more clefts. This review examines these approaches, and highlights key contributions to the field.

A Truncated Molecular Star

A pentanuclear coordination complex assembled from any palladium(II) component and non-chelating ligands is hitherto unreported. The pentanuclear complex [Pd₅ (L1)₅ (L2)₅ ](BF₄ )₁₀ , 1 reported here was prepared by the spontaneous complexation of [Pd(DMSO)₄ ](BF₄ )₂ with the non-chelating bidentate ligands 1,4-phenylenebis(methylene) diisonicotinate, L1 and 4,4'-bipyridine, L2 in a one-pot method at room temperature. The planar polycyclic complex 1 with outer diameters of ≈3 nm is termed as a "molecular star" owing to its resemblance with a pentagram shape. Interim paths leading to the star were also probed to decipher related dynamics of the system.

Modeling Coordination-Directed Self-Assembly of M2L4 Nanocapsule Featuring Competitive Guest Encapsulation

Exploring the mechanism of self-assembly and guest encapsulation of nanocapsules is highly imperative for the design of sophisticated molecular containers and multistimuli-responsive functional materials. Here we present a molecular dynamics simulation protocol with implicit solvent and simulated annealing techniques to investigate the self-assembly and competitive guest (C₆₀ and C₇₀ fullerenes) encapsulation of a M₂L₄ nanocapsule that is self-assembled by the coordination of mercury cations and bent bidentate ligands. Stepwise formation of the nanocapsule and competitive fullerene encapsulation during dynamic structural changes in the self-assembly were detected successfully. Such processes were driven by coordination bonding and π-π stacking and obey the minimum total potential energy principle. Potential of mean force calculations for guest binding to the M₂L₄ nanocapsule explained the mechanism underlying the competitive encapsulations of C₆₀ and C₇₀. This work helps design new functional nanomaterials capable of guest encapsulation and release.

Functional metallosupramolecular architectures using 1,2,3-triazole ligands: it's as easy as 1,2,3 “click”

Self-assembled metallosupramolecular architectures generated using “click” ligands have become an increasingly popular area of inorganic chemistry.

A self-assembled M2L4 cage incorporating electron-rich 9-(1,3-dithiol-2-ylidene)fluorene units

An electron-rich M2L4 cage is depicted, in which the four peripheral redox-active ligands (L) are 9-(1,3-dithiol-2-ylidene)fluorene units.