Chemorheological Monitoring of Cross-Linking in Slide-ring Gels Derived From α-cyclodextrin Polyrotaxanes

Despite hundreds of studies involving slide-ring gels derived from cyclodextrin (CD)-based polyrotaxanes (PRs), their covalent cross-linking kinetics are not well characterized. We employ chemorheology as a tool to measure the gelation kinetics of a model slide-ring organogel derived from α-cyclodextrin/poly (ethylene glycol) PRs cross-linked with hexamethylenediisocyanate (HMDI) in DMSO. The viscoelastic properties of the gels were monitored in situ by small-amplitude oscillatory shear (SAOS) rheology, enabling us to estimate the activation barrier and rate law for cross-linking while mapping experimental parameters to kinetics and mechanical properties. Gelation time, gel point, and final gel elasticity depend on cross-linker concentration, but polyrotaxane concentration only affects gelation time and elasticity (not gel point), while temperature only affects gelation time and gel point (not final elasticity). These measurements facilitate the rational design of slide-ring networks by simple parameter selection (temperature, cross-linker concentration, PR concentration, reaction time)

Tunable solid-state fluorescent materials for supramolecular encryption

Tunable solid-state fluorescent materials are ideal for applications in security printing technologies. A document possesses a high level of security if its encrypted information can be authenticated without being decoded, while also being resistant to counterfeiting. Herein, we describe a heterorotaxane with tunable solid-state fluorescent emissions enabled through reversible manipulation of its aggregation by supramolecular encapsulation. The dynamic nature of this fluorescent material is based on a complex set of equilibria, whose fluorescence output depends non-linearly on the chemical inputs and the composition of the paper. By applying this system in fluorescent security inks, the information encoded in polychromic images can be protected in such a way that it is close to impossible to reverse engineer, as well as being easy to verify. This system constitutes a unique application of responsive complex equilibria in the form of a cryptographic algorithm that protects valuable information printed using tunable solid-state fluorescent materials

Post-synthesis modification of slide-ring gels for thermal and mechanical reconfiguration

Ring-sliding behavior in polyrotaxanes imbues gels, elastomers, and glasses with remarkable stress-dissipation and actuation properties. Since these properties can be modulated and tuned by structural parameters, many efforts have been devoted to developing synthetic protocols that define the structures and properties of slide-ring materials. We introduce post-synthetic modifications of slide-ring gels derived from unmodified &alpha;-cyclodextrin and poly(ethylene glycol) polyrotaxanes that enable (i) actuation and control of the thermo-responsive lower critical solution temperature (LCST) behavior of ring-modified slide-ring hydrogels, and (ii) chemically bonding separate gels into hybrid or shape-reconfigured macro-structures with a slide-ring adhesive solution. The mechanical properties of the post-modified gels have been characterized by shear rheology and uniaxial tensile tests, while the corresponding xerogels were characterized by wide-angle X-ray scattering. These demonstrations show that post-synthetic modification offers a practical solution for re-configuring the properties and shapes of slide-ring gels. &nbsp;</p

Ex^2Box: Interdependent Modes of Binding in a Two-Nanometer-Long Synthetic Receptor

Incorporation of two biphenylene-bridged 4,4′-bipyridinium extended viologen units into a para-phenylene-based cyclophane results in a synthetic receptor that is 2 nm long and adopts a box-like geometry. This cyclophane, Ex^2Box^4+, possesses the ability to form binary and ternary complexes with a myriad of guest molecules ranging from long π-electron-rich polycyclic aromatic hydrocarbons, such as tetracene, tetraphene, and chrysene, to π-electron-poor 2,6-dinitrotoluene, 1,2,4-trichlorobenzene, and both the 9,10- and 1,4-anthraquinone molecules. Moreover, Ex^2Box^4+ is capable of forming one-to-one complexes with polyether macrocycles that consist of two π-electron-rich dioxynaphthalene units, namely, 1,5-dinaphtho[38]crown-10. This type of broad molecular recognition is possible because the electronic constitution of Ex^2Box^4+ is such that the pyridinium rings located at the “ends” of the cyclophane are electron-poor and prefer to enter into donor–acceptor interactions with π-electron-rich guests, while the “middle” of the cyclophane, consisting of the biphenylene spacer, is more electron-rich and can interact with π-electron-poor guests. In some cases, these different modes of binding can act in concert to generate one-to-one complexes which possess high stability constants in organic media. The binding affinity of Ex^2Box^4+ was investigated in the solid state by way of single-crystal X-ray diffraction and in solution by using UV–vis and NMR spectroscopy for 12 inclusion complexes consisting of the tetracationic cyclophane and the corresponding guests of different sizes, shapes, and electronic compositions. Additionally, density functional theory was carried out to elucidate the relative energetic differences between the different modes of binding of Ex^2Box^4+ with anthracene, 9,10-anthraquinone, and 1,4-anthraquinone in order to understand the degree with which each mode of binding contributes to the overall encapsulation of each guest

Good to be bad : should we be worried by the sharing economy?

We develop the notion of a legitimacy tipping point to demonstrate how informal economy practices are being utilized by innovative sharing economy ventures to gain a competitive advantage that is subsequently leveraged to reconfigure formal institutional arrangements. Companies who are able to scale rapidly can afford to contravene regulations, provided they have public support. When they reach a certain size, in terms of investment and customer numbers, regulators are forced into a reactive position where novel business models are legitimized. This raises an important question for regulators and entrepreneurs as to whether subverting business regulation is being viewed as a viable source of competitive advantage by scaling firms

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host&ndash;guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host&ndash;guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD &ldquo;bead&rdquo; gives rise to a symmetry-breaking effect when it is threaded onto a molecular &ldquo;string&rdquo;; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials

Dynamic force spectroscopy of synthetic oligorotaxane foldamers

Wholly synthetic molecules involving both mechanical bonds and a folded secondary structure are one of the most promising architec- tures for the design of functional molecular machines with un- precedented properties. Here, we report dynamic single-molecule force spectroscopy experiments that explore the energetic details of donor–acceptor oligorotaxane foldamers, a class of molecular switches. The mechanical breaking of the donor–acceptor interactions responsible for the folded structure shows a high constant rupture force over a broad range of loading rates, covering three orders of magnitude. In comparison with dynamic force spectroscopy performed during the past 20 y on various (bio)molecules, the near-equilibrium regime of oligorotaxanes persists at much higher loading rates, at which bio- molecules have reached their kinetic regime, illustrating the very fast dynamics and remarkable rebinding capabilities of the intramolecular donor–acceptor interactions. We focused on one single interaction at a time and probed the stochastic rupture and rebinding paths. Using the Crooks fluctuation theorem, we measured the mechanical work produced during the breaking and rebinding to determine a free- energy difference, ΔG, of 6 kcal·mol−1 between the two local confor- mations around a single bond