Inclusion of neutral guests by water-soluble macrocyclic hosts – a comparative thermodynamic investigation with cyclodextrins, calixarenes and cucurbiturils

<p>The driving forces of association between three different families of macrocycles as hosts, namely cyclodextrins (<i>α</i>-, <i>β</i>-, and <i>γ</i>-), <i>p</i>-sulfonatocalix[<i>n</i>]arenes (<i>n</i> = 4–6) as well as cucurbit[<i>n</i>]urils (<i>n</i> = 6–8), and three different bicyclic azoalkane homologues as guests, namely 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH), 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as well as 2,3-diazabicyclo[2.2.3]non-2-ene (DBN), were examined by means of calorimetric titrations, NMR spectroscopy and molecular dynamics simulation, all in aqueous solution. The small, spherical and uncharged guests preferably bind inside the cavities of the medium sized hosts. The inclusion complexation by <i>β</i>-cyclodextrin and <i>p</i>-sulfonatocalix[4]arene shows medium binding affinities (millimolar), while cucurbit[7]uril macrocycle shows very strong binding (micromolar). For all types of macrocycles, the complex formation is enthalpically driven (Δ<i>H</i>° < 0), accompanied by slightly unfavourable entropy changes (Δ<i>S</i>° < 0). The results are discussed in terms of the flexibility of the hosts, the hydrophobic character of their cavities and the release of high-energy water upon binding, and generalised by including two additional guests, the ketones cyclopentanone and (+)-camphor.</p

Efficient Host–Guest Energy Transfer in Polycationic Cyclophane–Perylene Diimide Complexes in Water

We report the self-assembly of a series of highly charged supramolecular complexes in aqueous media composed of cyclobis­(4,4′-(1,4-phenylene)­bispyridine-<i>p</i>-phenylene)­tetrakis­(chloride) (ExBox) and three dicationic perylene diimides (PDIs). Efficient energy transfer (ET) is observed between the host and guests. Additionally, we show that our hexacationic complexes are capable of further complexation with neutral cucurbit[7]­uril (CB[7]), producing a 3-polypseudorotaxane via the self-assembly of orthogonal recognition moieties. ExBox serves as the central ring, complexing to the PDI core, while two CB[7]­s behave as supramolecular stoppers, binding to the two outer quaternary ammonium motifs. The formation of the 3-polypseudorotaxane results in far superior photophysical properties of the central PDI unit relative to the binary complexes at stoichiometric ratios. Lastly, we also demonstrate the ability of our binary complexes to act as a highly selective chemosensing ensemble for the neurotransmitter melatonin