Thermodynamic Analysis of Allosteric and Chelate Cooperativity in Di- and Trivalent Ammonium/Crown-Ether Pseudorotaxanes.

A detailed thermodynamic analysis of the axle-wheel binding in di- and trivalent secondary ammonium/[24]crown-8 pseudorotaxanes is presented. Isothermal titration calorimetry (ITC) data and double mutant cycle analyses reveal an interesting interplay of positive as well as negative allosteric and positive chelate cooperativity thus providing profound insight into the effects governing multivalent binding in these pseudorotaxanes.

Chelate cooperativity effects on the formation of di- and trivalent pseudo[2]rotaxanes with diketopiperazine threads and tetralactam wheels.

The formation of singly, doubly and triply threaded pseudo[2]rotaxanes with diketopiperazine threads and tetralactam wheels is investigated with respect to chelate cooperativity effects on multivalent binding. Two series of guest molecules are prepared which differ with respect to their spacers, one with preorganised centrepieces with di- or tripodal roof-like structures, one with more flexible spacers. The thermodynamics of pseudorotaxane formation is examined using isothermal titration calorimetry and (1)H NMR spectroscopy. Force-field calculations provide more detailed structural insight and help rationalizing the thermodynamic data. All di- and trivalent pseudorotaxanes exhibit positive chelate cooperativity presumably arising from spacer-spacer interactions. Higher cooperativity factors are observed for the more preorganised threads.

Gating the photochromism of an azobenzene by strong host-guest interactions in a divalent pseudo[2]rotaxane.

The ability of an E-configured azobenzene guest to undergo photoisomerisation is controlled by the presence of a complementary host. Addition of base/acid allowed for a weakening/strengthening of the interactions in the divalent pseudo[2]rotaxane complex and hence could switch on/off photochromic activity.

Self-recovering stimuli-responsive macrocycle-equipped supramolecular ionogels with unusual mechanical properties.

A chiral, crown-ether-functionalized bisurea gelator forms supramolecular gels in ionic liquids. The resulting ionogels show a remarkably high thermal stability with gel-sol transition temperatures (T(gs)) reaching more than 100 °C. The mechanical strength of these ionogels is surprisingly high and even comparable to that of cross-linked protein fibres. Furthermore, the ionogels exhibit rapid self-recovery properties after structural damage caused by deformation. Pseudorotaxanes form from the gelators' benzo[21]crown-7 ethers as the wheels and secondary ammonium ions as the axles despite the competition between that cation and the imidazolium ions of the ionic liquid for crown ether binding. Pseudorotaxane formation as an external chemical stimulus triggers the gel-sol transition of the ionogels.