pH-Switched fluorescent pseudo rotaxane assembly of cucurbit[7]uril with bispyridinium ethylene derivatives

The Diversity of Cucurbituril Molecular Switches and Shuttles

Ring translocation switches and shuttles featuring a macrocycle (or a ring molecule) navigating between two or more stations continue to attract attention. While the vast majority of these systems are developed in organic solvents, the cucurbituril (CB) macrocycles are ideally suited to prepare such systems in water. Indeed, their stability and their relatively high affinity for relevant guest molecules are key attributes toward translating the progresses made in organic solvents, into water. This concept article summarizes the findings, key advances and multiple possibilities offered by CBs toward advanced molecular switches and shuttles in water.

A light-responsive molecular switch based on cucurbit[7]uril and 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium dibromide displaying white light emission

By using ¹H NMR, ESI-MS and UV spectra, a novel light-responsive molecular switch constructed using 1,1'-bis(benzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium (1²⁺) and cucurbit[7]uril (Q[7]) is demonstrated. The E- to Z-isomerization of the double bond in 1²⁺ results in the transition of the switching states from the 1 : 2 complex E-1²⁺@Q[7]₂ to the stable 1 : 1 complex Z-1²⁺@Q[7]. In particular, both the 1 : 2 complex and the 1 : 1 complex can emit cold white fluorescence under UV light.

A p-tert-Tutyldihomooxacalix[4]arene Based Soft Gel for Sustained Drug Release in Water

P-tert-butyldihomooxacalix[4]arene is a well-known calix[4]arene analog in which one CH₂ bridge is replaced by one -O- group. Thus, dihomooxacalix[4]arene has a slightly larger cavity than that of calix[4]arene and usually possesses a more flexible cone conformation, and the bridged oxygen atom might provide additional binding sites. Here, we synthesized a new functional p-tert-butyldihomooxacalix[4]arene 1 through Ugi reaction with good yield (70%), starting from condensed p-tert-butyldihomooxacalix[4]arene O-alkoxy-substituted benzaldehydes, benzoic acid, benzylamine, and cyclohexyl isocyanide. Proton nuclear magnetic resonance spectroscopy (¹H NMR), ¹³C NMR, IR, and diffusion-ordered ¹H NMR spectroscopy (DOSY) methods were used to characterize the structure of 1. Then soft gel was prepared by adding 1 into cyclohexane directly. It shows remarkable thermoreversibility and can be demonstrated for several cycles. As is revealed by scanning electron microscopy (SEM) images, xerogel showed highly interconnected and homogeneous porous network structures, and hence, the gel is suitable for storage and controlled release.

Tunable Fluorescence-Responsive Double Hydrophilic Block Polymers Induced by the Formation of Pseudopolyrotaxanes with Cucurbit[7]Uril

There is an urgent need for new strategies that allow the simultaneous detection and control of drug delivery. By making use of supramolecular host-guest interactions, a kind of pseudopolyrotaxanes, as a visualizable nanoscale drug carrier has been constructed by self-assembly of cucurbit[7]uril (CB[7]) with methoxy poly(ethylene glycol)-block-quaternized poly(4-vinyl pyridine) (mPEG-b-QP4VP) using 4-(chloromethyl)benzonitrile. Simple addition of CB[7] into an aqueous solution of mPEG-b-QP4VP resulted in noncovalent attachment of CB[7] to 4-cyanobenzyl-containing polymers, transforming the nonemissive mPEG-b-QP4VP micelles into highly fluorescent micelles. These pseudopolyrotaxanes micelles exhibited remarkable supramolecular assembly-induced emission enhancement and excellent biocompatibility, showing great potential for bioimaging applications. In addition, the efficient cellular uptake of the developed pseudopolyrotaxanes micelles loaded with the anticancer drug doxorubicin was a promising platform for simultaneous cell imaging and drug delivery, thereby widening their application in cancer theranostics.

Host-guest interactions in nor-seco-cucurbit[10]uril: novel guest-dependent molecular recognition and stereoisomerism

The unique monomer and excimer fluorescence emissions of pyrene were first exploited as distinctly photophysical signals to identify the possible diastereomers of guests within nor-seco-cucurbit[10]uril (NS-CB[10]) cavities. Further experiments revealed that balancing the hydrophilic and hydrophobic effects of the guest in aqueous solution can improve the molecular recognition and binding ability of NS-CB[10].

Synthesis of a water-soluble 2,2'-biphen[4]arene and its efficient complexation and sensitive fluorescence enhancement towards palmatine and berberine

A water-soluble 2,2′-biphen[4]arene (2,2’-CBP4) containing eight carboxylato moieties was synthesized and characterized. Its complexation behavior towards two alkaloids, palmatine (P) and berberine (B), was investigated by means of fluorescence and ¹H NMR spectroscopy in aqueous phosphate buffer solution (pH 7.4). In the presence of 2,2’-CBP4, ¹H NMR signals of P and B displayed very large upfield shifts, indicating the formation of inclusion complexes with strong binding affinities. Fluorescence titration experiments showed that P and B exhibited dramatic fluorescence enhancement of more than 600 times upon complexation with 2,2’-CBP4. Particularly, the fluorescence intensity is strong enough to be readily distinguished by the naked eye. Although the two guests have similar structures, the association constant of B with 2,2’-CBP4 (K_a = (2.29 ± 0.27) × 10⁶ M⁻¹) is 3.9 times larger than that of P (K_a = (5.87 ± 0.24) × 10⁵ M⁻¹).