Precise Rate Control of Pseudorotaxane Dethreading by pH-Responsive Selectively Functionalized Cyclodextrins

Two Squares in a Barrel: An Axially Disubstituted Conformationally Rigid Aliphatic Binding Motif for Cucurbit[6]uril

Novel binding motifs suitable for the construction of multitopic guest-based molecular devices (e.g., switches, sensors, data storage, and catalysts) are needed in supramolecular chemistry. No rigid, aliphatic binding motif that allows for axial disubstitution has been described for cucurbit[6]uril (CB6) so far. We prepared three model guests combining spiro[3.3]heptane and bicyclo[1.1.1]pentane centerpieces with imidazolium and ammonium termini. We described their binding properties toward CB6/7 and α-/β-CD using NMR, titration calorimetry, mass spectrometry, and single-crystal X-ray diffraction. We found that a bisimidazolio spiro[3.3]heptane guest forms inclusion complexes with CB6, CB7, and β-CD with respective association constants of 4.0 × 104, 1.2 × 1012, and 1.4 × 102. Due to less hindering terminal groups, the diammonio analogue forms more stable complexes with CB6 (K = 1.4 × 106) and CB7 (K = 3.8 × 1012). The bisimidazolio bicyclo[1.1.1]pentane guest forms a highly stable complex only with CB7 with a K value of 1.1 × 1011. The high selectivity of the new binding motifs implies promising potential in the construction of multitopic supramolecular components.

A Multiresponsive Calix[6]arene Pseudorotaxane Empowered by Fluorophoric Dansyl Groups

We report the synthesis and characterization, by means of NMR and UV-visible spectroscopy and electrochemical techniques, of a dansyl calix[6]arene derivative and of its pseudorotaxane complex with a bipyridinium-based axle. This novel macrocycle shows remarkable complexation ability, in analogy with parent compounds, while the dansyl moieties impart valuable features to the system. Indeed, these units: i) signal the state of the system by fluorescence; ii) can be reversibly protonated, enabling the modulation of the complexation abilities of the macrocycle; iii) participate in photoinduced electron transfer processes, which may be exploited to tune the stability of the supramolecular complex. Therefore, in this multiresponsive pseudorotaxane, the threading and de-threading motions of the molecular components can be modulated either by protonation of the calixarene host or by reduction of the bipyridinium guest, which can be accomplished both by electrochemical reduction and via photoinduced electron transfer. Overall, three orthogonal and reversible stimuli can be used to induce molecular movements of the pseudorotaxane components.

Multicharged cyclodextrin supramolecular assemblies

Multicharged cyclodextrin (CD) supramolecular assemblies, including those based on positively/negatively charged modified mono-6-deoxy-CDs, per-6-deoxy-CDs, and random 2,3,6-deoxy-CDs, as well as parent CDs binding positively/negatively charged guests, have been extensively applied in chemistry, materials science, medicine, biological science, catalysis, and other fields. In this review, we primarily focus on summarizing the recent advances in positively/negatively charged CDs and parent CDs encapsulating positively/negatively charged guests, especially the construction process of supramolecular assemblies and their applications. Compared with uncharged CDs, multicharged CDs display remarkably high antiviral and antibacterial activity as well as efficient protein fibrosis inhibition. Meanwhile, charged CDs can interact with oppositely charged dyes, drugs, polymers, and biomacromolecules to achieve effective encapsulation and aggregation. Consequently, multicharged CD supramolecular assemblies show great advantages in improving drug-delivery efficiency, the luminescence properties of materials, molecular recognition and imaging, and the toughness of supramolecular hydrogels, in addition to enabling the construction of multistimuli-responsive assemblies. These features are anticipated to not only promote the development of CD-based supramolecular chemistry but also contribute to the rapid exploitation of these assemblies in diverse interdisciplinary applications.

Metastable doubly threaded [3]rotaxanes with a large macrocycle

Ring size is a critically important parameter in many interlocked molecules as it directly impacts many of the unique molecular motions that they exhibit. Reported herein are studies using one of the largest macrocycles reported to date to synthesize doubly threaded [3]rotaxanes. A large ditopic 46 atom macrocycle containing two 2,6-bis(N-alkyl-benzimidazolyl)pyridine ligands has been used to synthesize several metastable doubly threaded [3]rotaxanes in high yield (65-75% isolated) via metal templating. Macrocycle and linear thread components were synthesized and self-assembled upon addition of iron(ii) ions to form the doubly threaded pseudo[3]rotaxanes that could be subsequently stoppered using azide-alkyne cycloaddition chemistry. Following demetallation with base, these doubly threaded [3]rotaxanes were fully characterized utilizing a variety of NMR spectroscopy, mass spectrometry, size-exclusion chromatography, and all-atom simulation techniques. Critical to the success of accessing a metastable [3]rotaxane with such a large macrocycle was the nature of the stopper group employed. By varying the size of the stopper group it was possible to access metastable [3]rotaxanes with stabilities in deuterated chloroform ranging from a half-life of <1 minute to ca. 6 months at room temperature potentially opening the door to interlocked materials with controllable degradation rates.

Size-dependent compression of threaded alkyldiphosphate in head to head cyclodextrin [3]pseudorotaxanes

The encapsulation of guests in a confined space enables unusual conformations and reactivities. In particular, the compression of akyl chains has been obtained by self-assembled molecular capsules but such an effect has not been reported in solution for pseudorotaxane architectures. By exploiting the tendency of cyclodextrin (CD) to form head to head [3]pseudorotaxanes and the hydrogen bonding abilities of phosphate groups, we have studied the effect of the CD dimer cavity on the conformation of threaded α,ω-alkyl-diphosphate axles. The formation of [2]pseudorotaxanes and [3]pseudorotaxanes was investigated by a combination of NMR, ITC and X-ray diffraction techniques. In the solid state, the [3]pseudorotaxane with a C₈ axle presents a fully extended conformation with both terminal phosphate groups interacting with hydroxyl groups of the primary rim of CDs. Such hydrogen bonding interactions are also present with the C₉ and C₁₀ axles resulting in a compression of the alkyl chain with gauche conformations in the solid state. NMR studies have shown that this effect is maintained in solution resulting in a size-dependent progressive compression of the alkyl chain by the CD [3]pseudorotaxane architecture for C₉, C₁₀ and C₁₁ axles.

Alkyl chain compression of alkanediphosphate guests was achieved by head-to-head cyclodextrin [3]pseudorotaxanes in a mechanostereoselective self-assembly process.