Cucurbit[n]urils (n = 5\u20138): a comprehensive solid state study

Cucurbit[n]urils (CB[n], n = 5\u20138) have been prepared, separated, and purified, and the structure of their solid state assemblies has been addressed. A number of general features were identified which are of interest to understand some peculiar properties of cucurbiturils (solubility, aggregation, assembly, transformation to porous crystals, influence of air humidity). CB[5], CB[6], and CB[8] were isolated as hydrate crystals whose structures were found to show a strong tendency of the macrocycles to interpenetrate. A self-closing effect was rationalized in terms of multiple weak CH\ub7\ub7\ub7O interactions between the macrocycles, the degree of which is discussed in detail. Solid state cross polarization magic angle spinning (CP-MAS) 13C NMR spectra obtained at 900 MHz were correlated with the crystal structures. An odd\u2013even effect in the crystallinity of thermally treated CB samples (CB[5] and CB[7] amorphous, CB[6] and CB[8] crystalline) was observed, which is reflected in differences in water solubility (CB[5] and CB[7] are water-soluble, whereas CB[6] and CB[8] are only very scarcely so). This may be explained by a less efficient self-association for CB[5] and CB[7] as compared with CB[6] and CB[8], which is reflected in the number of inter-cucurbituril CH\ub7\ub7\ub7O interactions per cucurbituril. This leads to a more favorable solvation for the CBs having an odd symmetry, whereas those with even symmetry prefer to self-associate in a manner ultimately leading to crystallization. We also propose an explanation for the presence of anions (Cl\u2013) inside some cucurbituril materials, whose cavity is often considered hydrophobic. Furthermore, it is demonstrated that large quantities of the very stable microporous CB[6] crystals (which have the R3\u305 channel structure) can be obtained very easily by a simple thermal treatment of the hexagonal crystals of CB[6] (P6/mmm structure) obtained directly from the initial reaction used to synthesize the various CB[n]. The micromorphology of the CB[n] powders was established using scanning electron microscopy (SEM), and the tendency of each material to absorb water from the atmosphere was demonstrated by thermogravimetric analyses (TGA).Peer reviewed: YesNRC publication: Ye

Interfacing supramolecular gels and quantum dots with ultrasound: smart photoluminescent dipeptide gels

Our recent discovery of a new dipeptide gelator that can be activated by ultrasound in alkanes and polar solvents[8] allows one to envision several unusual and unexpected applications. A striking feature of this dipeptide is the readily ultrasound-induced gelation in hexane containing CdSe/ZnS core/shell QDs. In this case, ultrasound allows a blurring of the frontier between \u2018\u2018soft\u2019\u2019 multi-responsive supramolecular gels and \u2018\u2018hard\u2019\u2019 light-emitting semiconductor NPs, affording new materials displaying properties from each component. Here, we describe the preparation of QD-dipeptide nanocomposite gels which show bright luminescence under UV light and which can be switched from liquid to solid by ultrasound and back to liquid when heated. Applications using the gel and the dry material (xerogel) are also demonstrated regarding chemical sensing.Peer reviewed: YesNRC publication: Ye

Photoluminescent quantum dot\u2013cucurbituril nanocomposites

The preparation of entrapped CdSe\u2013ZnS fluorescent quantum dots (QDs) in cucurbituril (CB) polymer capsules is reported.Peer reviewed: YesNRC publication: Ye