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

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.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 (Tgs) 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.DFG, SFB 765, Multivalenz als chemisches Organisations- und Wirkprinzip: Neue Architekturen, Funktionen und Anwendunge

Evaluation of multivalency as an organization principle for the efficient synthesis of doubly and triply threaded amide rotaxanes

Mono-, di- and trivalent pseudorotaxanes with tetralactam macrocycle hosts and axles containing diamide binding stations as the guests have been synthesised. Their threading behaviour was analyzed in detail by NMR experiments and isothermal titration calorimetry. An X-ray crystal structure of the monovalent pseudorotaxane confirms the binding motif. Double mutant cycle analysis provides the effective molarities and insight into the chelate cooperativity of multivalent binding. While the second binding event in a trivalent pseudorotaxane exhibits a slightly positive cooperativity, the third binding is nearly non-cooperative. Nevertheless, the enhanced binding affinities resulting from the multivalent interaction are the basis for a highly efficient synthesis of di- and trivalent rotaxanes through stoppering the axle termini by “click” chemistry. Evidence for the multiply threaded geometry comes from NMR spectroscopy as well as tandem mass-spectrometric fragmentation experiments of mass-selected rotaxane ions in the gas phase. Furthermore, the trivalent rotaxane can be controlled by external stimuli (chloride addition and removal) which lead to an elevator-type movement of the wheel along the axle

molecular recognition at interfaces

In order to investigate molecular recognition on surfaces, an azide- functionalized monolayer was deposited on gold. The monolayer was characterized by X-ray photoelectron spectroscopy (XPS) and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) experiments and the decomposition of the azide upon irradiation with X-ray beams was investigated. Subsequently, various alkyne-functionalized host and guest molecules were attached to the azide by 1,3-dipolar cycloaddition. These modified surfaces and their host–guest chemistry were analysed by XPS and angle-resolved NEXAFS. The reversibility of guest binding was shown for one example as a proof of principle

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.Peer Reviewe

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