Advancements and strategic approaches in catenane synthesis.

Catenanes, a distinctive category of mechanically interlocked molecules composed of intertwined macrocycles, have undergone significant advancements since their initial stages characterized by inefficient statistical synthesis methods. Through the aid of molecular recognition processes and principles of self-assembly, a diverse array of catenanes with intricate structures can now be readily accessed utilizing template-directed synthetic protocols. The rapid evolution and emergence of this field have catalyzed the design and construction of artificial molecular switches and machines, leading to the development of increasingly integrated functional systems and materials. This review endeavors to explore the pivotal advancements in catenane synthesis from its inception, offering a comprehensive discussion of the synthetic methodologies employed in recent years. By elucidating the progress made in synthetic approaches to catenanes, our aim is to provide a clearer understanding of the future challenges in further advancing catenane chemistry from a synthetic perspective.

Fabrication of Azacrown Ether-Embedded Covalent Organic Frameworks for Enhanced Cathode Performance in Aqueous Ni-Zn Batteries.

Crown ethers (CEs), known for their exceptional host-guest complexation, offer potential as linkers in covalent organic frameworks (COFs) for enhanced performance in catalysis and host-guest binding. However, their highly flexible conformation and low symmetry limit the diversity of CE-derived COFs. Here, we introduce a novel C3-symmetrical azacrown ether (ACE) building block, tris(pyrido)[18]crown-6 (TPy18C6), for COF fabrication (ACE-COF-1 and ACE-COF-2) via reticular synthesis. This approach enables precise integration of CEs into COFs, enhancing Ni2+ ion immobilization while maintaining crystallinity. The resulting Ni2+-doped COFs (Ni@ACE-COF-1 and Ni@ACE-COF-2) exhibit high discharge capacity (up to 1.27 mAh·cm-2 at 8 mA·cm-2) and exceptional cycling stability (>1000 cycles) as cathode materials in aqueous alkaline nickel-zinc batteries. This study serves as an exemplar of the seamless integration of macrocyclic chemistry and reticular chemistry, laying the groundwork for extending the macrocyclic-synthon driven strategy to a diverse array of COF building blocks, ultimately yielding advanced materials tailored for specific applications.

Molecular Hinges by Fusion of Pyrrole and Dithiin: Synthesis, Structure, Redox Chemistry and Host-Guest Complexation of Dipyrrolo-1,4-dithiins.

A series of novel hinge-like molecules, namely dipyrrolo-1,4-dithiins (PDs), were prepared and fully characterized by NMR, UV/vis, cyclic voltammogram, ESR, and single crystal X-ray diffraction (SCXRD) analysis. The lateral fusion of pyrroles with 1,4-dithiins has led to not only retained key features of a dithiin, but also enhanced redox-activity with increased susceptibility to radical cations via redox or chemical oxidation. Stabilization of their radicals are observed for the N,N-tert-butyl or N,N-triphenylmethyl PD as evidenced by ESR measurements. DFT calculations and SCXRD analysis revealed PDs are extremely flexible with adaptive molecular geometries that can be mechanically regulated via crystal packing or host-guest complexation. The excellent donor nature of PDs renders inclusion complexes with the cyclophane bluebox (cyclobis(paraquat-p-phenylene)), featuring association constants up to 104  M-1 . Additionally, a planarized transition intermediate associated with inversion dynamics of a PD has been preserved in the pseudorotaxane structure with assistances of π⋅⋅⋅π and S⋅⋅⋅π interactions. The hinged structure, excellent redox-activity, and adaptive nature of PDs could further enable accesses to exotic redox switchable host-guest chemistry and functional materials.

Cyclization of an Achiral Flipping Panel to Homochiral Tubes Exhibiting Circularly Polarized Luminescence.

Macrocyclization of the bendable 2,7-dimethoxythianthrene with methylene linkages afforded a pair of homochiral macrocycles featuring a hex-nut-like geometry. Their structures were fully characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction analysis. Their stable planar chirality facilitates efficient resolution of the pair of enantiomers which could be readily derivatized. Installing phenylethynyl groups on their rims leads to luminescent tubular macrocycles exhibiting circularly polarized luminescence with a large dissymmetry value |glum | of 5×10-3 . Supramolecular binding of electron-deficient guests by the tube results in fluorescence quenching, which proved its potential for the future development of switchable chiroptical systems.

Precise Control of Radial Catenane Synthesis via Clipping and Pumping.

An unprecedented molecular pumping cassette was designed and implemented for the construction of molecular necklaces, that is, radial [n]catenanes. The mechanism was fully confirmed on a model [2]pseudorotaxane, and the novel clipping-followed-by-pumping strategy was used to prepare a series of [n]catenanes (n = 2-5). A pair of [3]catenane diastereomers sequentially threaded with two different wheels was also accomplished. The success of utilizing molecular pumping to construct molecular necklaces offers new insights into complex molecular architectures and expands the application of molecular machines in synthesis.

Exploring the dynamics of Zr-based metal-organic frameworks containing mechanically interlocked molecular shuttles.

Zr(iv) metal-organic frameworks (MOFs) UiO-68 and PCN-57, containing triphenylene dicarboxylate (TPDC) and tetramethyl-triphenylene dicarboxylate (TTDC) linkers, respectively, were doped with an H-shaped tetracarboxylate linker that contains a [2]rotaxane molecular shuttle. The new MOFs, UWDM-8 and UWDM-9, contain a [2]rotaxane crossbar spanning the tetrahedral cavities of the fcu topology while the octahedral cavities remain empty. 13C solid-state NMR (SSNMR) spectra and solution 1H NMR spectra verified that the [2]rotaxanes were included as designed. Variable-temperature (VT) cross polarization (CP) magic-angle spinning (MAS) 13C SSNMR was used to explore the translational motion of the macrocyclic ring in both MOFs. The SSNMR results clearly show that the structure of the linker (TPDCvs.TTDC) affects the shuttling rate of the macrocyclic ring, although questions remain as to how rotation of the central phenylene unit of the strut might also affect the motion of the macrocycle.

Sulphur-Embedded Hydrocarbon Belts: Synthesis, Structure and Redox Chemistry of Cyclothianthrenes.

Cyclothianthrenes, a series of sulphur-embedded hydrocarbon belts proposed a decade ago, were successfully constructed through a stepwise bottom-up synthesis. The belt [6]cyclothianthrene ([6]CT) is the smallest and most strained member of the family yet reported. Both [6]CT and [8]CT are the first examples of cyclothianthrene characterized by single crystal X-ray diffraction. An unprecedented chiral belt [7]CT and a Möbius-shaped [9]CT were also achieved via modular synthesis. Crystallographic and computational studies show that belts [6]CT-[8]CT have prism-like conformations with well-defined tubular cavities which have potential for guest molecule inclusion. Cyclic voltammograms further revealed that these belts are redox-active. The success of constructing sulphur-embedded hydrocarbon belts, that is, cyclothianthrenes, greatly enriches the chemistry of heteroatom-doped molecular belts and tubes.

Highly Emissive Perylene Diimide-Based Metallacages and Their Host-Guest Chemistry for Information Encryption.

Here we report two highly emissive perylene diimide (PDI)-based metallacages and explore their complexation with polycyclic aromatic hydrocarbons, such as pyrene, triphenylene, and perylene. The fluorescence quantum yields of metallacages exceed 90% and their binding constants with perylene can reach as high as 2.41 × 104 M-1 in acetonitrile. These features enable further tuning of the emission of the host-guest complexes to obtain white-light emission based on the complementary orange emission of the metallacages and the blue emission of perylene. Moreover, owing to the huge differences of their quantum yields in solution and in the solid state, the host-guest complexes are successfully employed for information encryption. This study offers a general approach for the construction of emissive metallacages and explores their application for information encryption.

Optical Distinction between "Slow" and "Fast" Translational Motion in Degenerate Molecular Shuttles.

A series of six [2]rotaxane molecular shuttles was designed which contain an axle with a benzo-bis(imidazole) core (in either a neutral or dicationic form) and a single 24-membered, crown ether wheel (24C6, B24C6, or DMB24C6), and the shuttling rates of the ring along the axle were determined. The charged versions showed much slower shuttling rates as a result of the increase in noncovalent interactions between the axle and wheel. The [2]rotaxane with a B24C6 wheel shows a difference in fluorescence between the charged and neutral species, while the [2]rotaxane with a DMB24C6 wheel exhibits a difference in color between the charged and neutral compounds. These changes in optical properties can be attributed to the structural differences in the co-conformations of the [2]rotaxane as they adapt to the changes in acid/base chemistry. This allowed the relative rate of the translational motion of a molecular shuttle to be determined by observation of a simple optical probe.

From Binuclear Complexes to Molecular Necklaces: Incorporating Flexible Ligands into Rotaxanes.

The conversion of binuclear complexes into larger molecular necklaces can be achieved through rigidifying flexible ligands by threading them through a crown ether to form either an interpenetrated [2]pseudorotaxane or a permanently interlocked [2]rotaxane. The resulting complexes and assemblies are characterized by (1) H and DOSY NMR in solution and single-crystal X-ray diffraction in the solid-state.

Rigid, Bistable Molecular Shuttles Combining T-shaped Benzimidazolium and Y-shaped Imidazolium Recognition Sites.

Two different recognition sites, one a T-shaped 2,4,7-triphenylbenzimidazolium and the other a Y-shaped 2,4,5-triphenylimidazolium are combined to construct a rigid bistable [2]rotaxane molecular shuttle on which the position of a 24-membered crown ether macrocycle can be controlled by acid-base chemistry. Molecular shuttling in both the neutral and dicationic versions were studied.

Mechanically Interlocked Linkers inside Metal-Organic Frameworks: Effect of Ring Size on Rotational Dynamics.

A series of metal-organic framework (MOF) materials has been prepared, each containing a mechanically interlocked molecule (MIM) as the linker and a copper(II) paddlewheel as the secondary building unit (SBU). The MIM linkers are [2]rotaxanes with varying sizes of crown ether macrocycles ([22]crown-6, 22C6; [24]crown-6, 24C6; [26]crown-6, 26C6; benzo[24]crown-6, B24C6) and an anilinium-based axle containing four carboxylate donor groups. Herein, the X-ray structures of MOFs UWCM-1 (no crown) and UWDM-1(22) are compared and demonstrate the effect of including a macrocycle around the axle of the linker. The rotaxane linkers are linear and result in nbo-type MOFs with void space that allows for motion of the interlocked macrocycle inside the MOF pores, while the macrocycle-free linker is bent and yields a MOF with a novel 12-connected bcc structure. Variable temperature (2)H solid-state nuclear magnetic resonance showed that the macrocycles in UWDM-1(22), UWDM-1(24), and UWDM-1(B24) undergo different degrees and rates of rotation depending on the size and shape of the macrocycle.

Metal-organic frameworks with mechanically interlocked pillars: controlling ring dynamics in the solid-state via a reversible phase change.

Metal-organic framework (MOF) materials have been prepared that contain a mechanically interlocked molecule (MIM) as the pillaring strut between two periodic Zn-carboxylate layers. The MIM linker is a [2]rotaxane with a [24]crown-6 (24C6) macrocycle and an aniline-based axle with terminal pyridine donor groups. The single-crystal X-ray structures of MOFs UWDM-2 (1,4-diazophenyl-dicarboxylate) and UWDM-3 (1,4-biphenyl-dicarboxylate) show that both frameworks are large enough to contain the free volume required for rotation of the interlocked 24C6 macrocycle, but the frameworks are interpenetrated (UWDM-2, three-fold, and UWDM-3, two-fold). In particular, for UWDM-3 the 24C6 rings of the pillaring MIM are positioned directly inside the square openings of neighboring zinc dicarboxylate layers. Variable-temperature (VT) (2)H SSNMR demonstrated that the 24C6 macrocycles in UWDM-2 and UWDM-3 can only undergo restricted motions related to ring flexibility or partial rotation but are incapable of undergoing free rotation. VT-powder X-ray diffraction studies showed that upon activation of UWDM-3, by removing solvent, a phase change occurs. The new ß-phase of UWDM-3 retained crystallinity, and (2)H SSNMR demonstrated that the 24C6 macrocyclic ring of the pillared MIM strut is now free enough to undergo full rotation. Most importantly, the phase change is reversible; the ß version of the MOF can be reverted to the original α state by resolvation, thus demonstrating, for the first time, that the dynamics of a MIM inside a solid material can be controlled by a reversible phase change.

Organizing mechanically interlocked molecules to function inside metal-organic frameworks.

The idea that the workings of molecular switches, motors, and machines based on mechanically interlocked molecules can be transferred into the solid state by using them as the building blocks of metal-organic framework materials is addressed. This involves an in-depth review and analysis of the chemistry of coordination polymers and metal-organic frameworks in which the linkers are rotaxanes and catenanes. To date, two types of materials have been prepared: (1) coordination polymers in which the interlocked components are part of a complex architecture but do not display large amplitude molecular motion or function and (2) those that clearly demonstrate some type of supramolecular quality (molecular recognition) or relative motion between interlocked components (dynamics) reminiscent of their solution counterparts. The latter can be thought of as prototypes of solid-state molecular machines. The possibility of creating more sophisticated, solid-state materials that have the full characteristics of molecular switches, motors, and machines and the way forward for this chemistry is also discussed.

[2]Pseudorotaxane formation between rigid Y-shaped 2,4,5-triphenylimidazolium axles and [24]crown-8 ether wheels.

A new templating motif for the formation of [2]pseudorotaxanes is described in which rigid, Y-shaped axles with an imidazolium core and aromatic substituents at the 2-, 4- and 5-positions interact with [24]crown-8 ether wheels ([24]crown-8 and dibenzo[24]crown-8). The Y-shape of the axle significantly enhances the association between axle and wheel when compared to simple imidazolium cations.

A π-extended molecular belt with selective binding capability for fullerene C70.

A molecular belt incorporating naphthalene moieties, featuring an ellipsoidal cavity, was precisely engineered through bottom-up synthesis. Its pre-arranged geometry exhibits excellent complementarity to fullerene C70, resulting in remarkable selective binding ability (K = 1.3 × 106 M-1) for C70 compared to C60 (K = 176 M-1), forming a 1 : 1 complex. This superiority was unequivocally demonstrated by the single crystal structure of the complex, which revealed outstanding concave-convex shape complementarity between the two components. This highlights the potential application of molecular belts in the purification and separation of fullerenes.

Engineering Nanobelt Structure via Sulphur and Oxygen Doping: Synthesis, Structural Characterization, and Complexation with Fullerenes.

This study explores the synthesis, structural characterization, and host-guest interactions of heteroatom bridged nanobelts, focusing on a cyclothianthrene nanobelt and a fused nanobelt incorporating thianthrene and phenoxathiin. Utilizing a cyclization-followed-by-bridging synthetic approach, both molecular belts were successfully synthesized, and their structures confirmed through NMR and MALDI-TOF-MS analysis. Crystallographic studies revealed that the cyclothianthrene nanobelt adopts an octagonal column-like conformation, while the hybrid belt forms an oval tub-shaped shape, both exhibiting distinct assembly motifs. The host-guest chemistry of these nanobelts was investigated with fullerenes (C60, C70, and PC61BM). The cyclothianthrene belt showed no interaction with these fullerenes, whereas the other belt demonstrated adaptive binding capabilities, forming stable complexes with C60 and C70 through π-π interactions and C‒H∙∙∙S hydrogen bonds. The binding constants indicated that the hybrid belt has a stronger affinity for C70 due to better size complementarity. Additionally, its interaction with PC61BM showcased a specific 1:1 binding mode despite exhibiting a smaller binding constant. This study underscores the impact of heteroatom incorporation on the structural and functional properties of nanobelts, offering insights for future molecular design strategies.

ProBox: A Rigid yet Dynamic Cyclophane Capable of Adaptive and Redox-Switchable Host-Guest Binding.

A pyrrolodithiin-derived box-like cyclophane (ProBox), featuring an adaptive geometry with stimuli-responsiveness, was designed and successfully constructed. The dynamic and foldable dithiin subunit endowed the cyclophane with a compressible cavity which can transform from a hex-nut geometry to a nearly rectangular box upon complexing guests with various sizes and shapes. The resulting pseudorotaxane complexes could be dethreaded via electrochemical oxidation. Such an adaptive cavity along with redox-switchable host-guest binding of ProBox could enable further applications in complex molecular switches and machines.

Repairing Avascular Meniscal Lesions by Recruiting Endogenous Targeted Cells Through Bispecific Synovial-Meniscal Aptamers.

BACKGROUND: Tissue engineering is a promising treatment option for meniscal lesions in the avascular area, but a favorable cell source and its utilization in tissue-engineered menisci remain uncertain. Therefore, a more controllable and convenient method for cell recruitment is required. HYPOTHESIS: Circular bispecific synovial-meniscal (S-M) aptamers with a gelatin methacryloyl (GelMA) hydrogel can recruit endogenous synovial and meniscal cells to the site of the defect, thereby promoting in situ meniscal regeneration and chondroprotection. STUDY DESIGN: Controlled laboratory study. METHODS: Synovial and meniscal aptamers were filtered through systematic evolution of ligands by exponential enrichment (SELEX) and cross-linked to synthesize the S-M aptamer. A GelMA-aptamer system was constructed. An in vitro analysis of the bi-recruitment of synovial and meniscal cells was performed, and the migration and proliferation of the GelMA-aptamer hydrogel were also tested. For the in vivo assay, rabbits (n = 90) with meniscal defects in the avascular zone were divided into 3 groups: repair with the GelMA-aptamer hydrogel (GelMA-aptamer group), repair with the GelMA hydrogel (GelMA group), and no repair (blank group). Regeneration of the repaired meniscus and degeneration of the cartilage were assessed by gross and histological evaluations at 4, 8, and 12 weeks postoperatively. The mechanical properties of repaired menisci were also evaluated. RESULTS: In vitro synovial and meniscal cells were recruited simultaneously by the S-M aptamer with high affiliation and specificity. The GelMA-aptamer hydrogel promoted the migration of targeted cells. Compared with the other groups, the GelMA-aptamer group showed enhanced fibrocartilaginous regeneration, lower cartilage degeneration, and better mechanical strength at 12 weeks after meniscal repair. CONCLUSION/CLINICAL RELEVANCE: Bispecific S-M aptamers could be used for avascular meniscal repair by recruiting endogenous synovial and meniscal cells and promoting fibrocartilaginous regeneration.

[2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane frameworks containing tetra-substituted dibenzo[24]crown-8 wheels.

[2]Pseudorotaxanes, [2]rotaxanes and metal-organic rotaxane framework materials that utilise DB24C8 as the wheel component are well known and structural variations based on changing the axle component are common. Studies in which the DB24C8 wheel is structurally modified are much more limited. Herein, is described the synthesis of symmetrical DB24C8 analogues containing four CH(2)OR (R = CH(2)CH(2)CH(3), CH(2)(C(6)H(5)), C(6)H(5) and C(6)H(4)(4-COOEt)) substituents on the 4 and 5 positions of the aromatic rings. The effect of these molecular appendages on the stability and structures of the interpenetrated and interlocked molecules derived from these new wheels is described. The major effects are an increase in association constants for the formation of [2]pseudorotaxanes relative to DB24C8, the crystal packing of [2]rotaxanes and a change on the internal structure of a 2D MORF (R = C(6)H(5)) compared to DB24C8.