Synthesis and supramolecular properties of glycoluril tetramer

Synthesis of glycoluril tetramer bearing hydrogen atoms on its convex face and framed by two xylylene units is reported. This tetramer forms dimeric aggregates in the solid state. Host–guest properties of the tetramer with various ammonium and pyridinium guests were determined using ¹H NMR spectroscopy. The results were compared with structurally related host molecules.     

Switching Properties of a Spiropyran–Cucurbit[7]uril Supramolecular Assembly: Usefulness of the Anchor Approach

A nitrospiropyran, which was modified with a cadaverine‐derived anchor, was investigated with respect to its thermally induced isomerizations, hydrolytic stability of the merocyanine form, and photochromic ring closure. The host–guest complexation of the anchor by the cucurbit[7]uril macrocycle, evidenced by absorption titration, NMR spectroscopy, and electrospray ionization mass spectrometry, produced significant improvements of the switching properties of the photochrome: 1) appearance of the merocyanine form about 70 times faster, 2) practically unlimited hydrolytic stability of the merocyanine (two and a half days without any measureable decay), and 3) fast, clean, and fatigue‐resistant photoinduced ring closure back to the spiro form. The importance of an adequate molecular design of the anchor was demonstrated by including control experiments with spiropyrans with a shorter linker or without such structural asset.     

Synthesis and Electronic Properties of 1,2‐Hemisquarimines and Their Encapsulation in a Cucurbit[7]uril Host

The synthesis of a new class of robust squaraine dyes, colloquially named 1,2‐hemisquarimines (1,2‐HSQiMs), through the microwave‐assisted condensation of aniline derivatives with the 1,2‐squaraine core is reported. In CH₃CN, 1,2‐HSQiMs show a broad absorption band with a high extinction coefficient and a maximum at around λ=530 nm, as well as an emission band centered at about λ=574 nm, that are pH dependent. Protonation of the imine nitrogen causes a redshift of both absorption and emission maxima, with a concomitant increase in the lifetime of the emitting excited state. Encapsulation of the chromophore into a cucurbit[7]uril host revealed fluorescence enhancement and increased photostability in water. The redox characteristics of 1,2‐HSQiMs indicate that charge injection into TiO₂ is possible; this opens up promising perspectives for their use as photosensitizers for solar energy conversion.     

Synthesis,characterisation and properties of star polypseudorotaxanes with cucurbit[7]uril

New star polypseudorotaxanes that include 4,4′-dipyridyl-terminated 4-arm-poly(ethylene glycol) (PC1) and cucurbit[7]uril (CB[7]) were easily synthesised. The ¹H NMR, UV–vis and resonance light scattering (RLS) data show that the viologen units of the star polypseudorotaxanes could fall off from the hydrophobic cavity of CB[7] with the addition of 1-adamantylamine (ADA). The cyclic voltammograms results indicate that the star polypseudorotaxanes have the redox property with the addition of ADA compared with the non-redox property of the pure star polypseudorotaxanes. The biological toxicity experiment shows that the bactericidal activity of the star polypseudorotaxanes in Escherichia coli was aroused by adjusting CB[7] dethreading from the star polypseudorotaxanes with the addition of another competitive guest ADA, which has a very high affinity for CB[7].     

Host–guest control on the formation of pinacyanol chloride H-aggregates in anionic polyelectrolyte solutions

The interactions of pinacyanol chloride (PIN), a cationic cyanine dye, with the anionic polyelectrolyte poly(acrylic acid, sodium salt) (PAA) towards enhancing H-aggregation were investigated by electronic absorption spectroscopy. We employed the cucurbit[7]uril (CB7) host to control the formation of these aggregates via host–guest binding interactions with the dye molecules. Absorption spectroscopic studies clearly demonstrate that PAA enhances the formation of PIN H-aggregates. Electrostatic interactions between the cyanine dye molecules and the polyelectrolyte chains assist the formation of H-aggregates at very low dye concentrations (ca. 10 μM). Furthermore, the presence of CB7 was found to effectively disrupt the interactions responsible for dye aggregation. Thus, CB7 completely disrupts the H-aggregates (as well as lower concentrations of J-aggregates) by forming inclusion complexes with PIN. A competing guest, 1-aminoadamantane (AD), was utilised to adjust the extent of aggregation of the cyanine dye. AD regulates aggregate formation by forming an extremely stable complex with CB7 and exerting a tight control on the CB7 concentration available to interact and bind with the dye. Our spectroscopic data clearly indicate that by varying the relative molar ratios of CB7 host, AD and polyelectrolyte acid groups, we can quantitatively control the extent of formation of PIN H-aggregates in these solutions.     

Modulation of Photophysics and pKa Shift of the Anti‐cancer Drug Camptothecin in the Nanocavities of Supramolecular Hosts

This article reports the pK_a shift of an anti‐cancer drug, 20(S)‐camptothecin (CPT), upon encapsulation into the nanocavity of a cucurbit[7]uril (CB7) macrocycle. Steady‐state, time‐resolved fluorescence and electrospray ionisation mass spectrometry (ESI‐MS) studies provide evidence for the formation of both 1:1 and 2:1 (CB7 ? CPT) stoichiometries. Astonishingly, we have found that protonation of CPT takes place at a higher concentration of macrocycle (≥50 μM ) when the 2:1 stoichiometric complex develops. However, we did not find any proof for protonation of CPT when it is encased by a β‐cyclodextrin cavity, which has a cavity size almost the same as that of CB7. Hence, we conclude that electron‐rich carbonyl portals of CB7 have an important role in protonation of the drug in the 2:1 inclusion complex. Docking and semi‐empirical quantum chemical calculations have been employed to gain an insight into the molecular picture of orientation of CPT in the inclusion complexes. It is clearly seen from the optimised structure of the 2:1 (CB7 ? CPT) inclusion complex that the quinoline nitrogen of CPT does not reside within either of the CB7 cavities, rather it is almost sandwiched between two CB7 rings, and therefore, it experiences huge electron density exerted by both carbonyl portals of the macrocycles. As a result, the pK_a of CPT shifts from 1.2 to 6.2. Finally, controlled release of the drug has been achieved through the introduction of NaCl, which is rich in cells, as an external stimulus. We hope this recognition‐mediated binding and release mechanism can be useful for activation of the drug and controlled release of the drug in therapeutic uses.