Ether Naphthotube Host-Guest Complexes and [2]Rotaxanes with Dications.

The interactions between ether naphthotube and a series of dication guests in organic solution were investigated. It was found that ether naphthotube formed stable host-guest complexes selectively with these guests in a 1 : 1 stoichiometric ratio with association constants ranging from 102 to 106 M-1, which were confirmed by 1H-NMR spectra and ITC experiments. The host-guest interactions are driven by enthalpy change as the entropic factors are unfavorable. Positive correlations between ΔH and ΔS have been observed in the host-guest complexes. Furthermore, the para-substitution of the guests can significantly affect the binding affinities through a combination of field/inductive and resonance effects by following a linear free energy relationship. Based on the host-guest complexes composed of ether naphthotube and organic cations, two interlocked [2]rotaxanes were prepared by cationization reaction and Huisgen cycloaddition between the cations and the stopper components. The ether naphthotube-based host-guest complexes are useful for creating sophisticated interlocked molecules.

Precise Recognition in Water by an Endo-Functionalized Cavity: Tuning the Complementarity of Binding Sites.

Precise binding towards structurally similar substrates is a common feature of biomolecular recognition. However, achieving such selectivity-especially in distinguishing subtle differences in substrates-with synthetic hosts can be quite challenging. Herein, we report a novel design strategy involving the combination of different rigid skeletons to adjust the distance between recognition sites within the cavity, which allows for the highly selective recognition of hydrogen-bonding complementary substrates, such as 4-chromanone. X-ray single-crystal structures and density functional theory calculations confirmed that the distance of endo-functionalized groups within the rigid cavity is crucial for achieving high binding selectivity through hydrogen bonding. The thermodynamic data and molecular dynamics simulations revealed a significant influence of the hydrophobic cavity on the binding affinity. The new receptor possesses both high selectivity and high affinity, which provide valuable insights for the design of customized receptors.

Chiral recognition of neutral guests by chiral naphthotubes with a bis-thiourea endo-functionalized cavity.

Developing chiral receptors with an endo-functionalized cavity for chiral recognition is of great significance in the field of molecular recognition. This study presents two pairs of chiral naphthotubes containing a bis-thiourea endo-functionalized cavity. Each chiral naphthotube has two homochiral centers which were fixed adjacent to the thiourea groups, causing the skeleton and thiourea groups to twist enantiomerically through chiral transfer. These chiral naphthotubes are highly effective at enantiomerically recognizing various neutral chiral molecules with an enantioselectivity up to 17.0. Furthermore, the mechanism of the chiral recognition has been revealed to be originated from differences in multiple non-covalent interactions. Various factors, such as the shape of cavities, substituents of guests, flexibility of host and binding modes are demonstrated to contribute to creating differences in the non-covalent interactions. Additionally, the driving force behind enantioselectivity is mainly attributed to enthalpic differences, and enthalpy -entropy compensation has also been observed to influence enantioselectivity.

Non-Equilibrium Kinetic States of a [2]Rotaxane-Based Molecular Shuttle Controlled by Acid Concentrations.

A [2]rotaxane-based molecular shuttle with an acid-responsive asymmetric macrocycle on a symmetric dumbbell axle is reported. Upon adding TFA, the macrocycle, namely the amine naphthotube, is protonated and translocates from the di(quaternary ammonium) station to the triazole stations because of electrostatic repulsion and weakened binding. The shuttling kinetics are slow due to the steric hindrance caused by the ethyl group on the quaternary ammonium center and can be followed by 1 H NMR spectroscopy. Interestingly, it was found that the shuttling kinetics depends on the concentration of TFA. A kinetic intermediate was detected and can even be captured in the presence of a high concentration of TFA. Extensive control experiments revealed that the shuttling kinetics and the capture of the kinetic intermediate are related to the different protonation states of the rotaxanes.

Biomimetic Synchronized Motion of Two Interacting Macrocycles in [3]Rotaxane-Based Molecular Shuttles.

Noncovalent interactions between all the neighboring components in biomolecular machines are responsible for their synchronized motion and thus complex functions. This strategy has rarely been used in multicomponent molecular machines. Here, we report four [3]rotaxane-based molecular shuttles. Noncovalent interactions among the three components (two interacting macrocycles and one axle) not only cause a "systems-level" effect on the relative positions of the two macrocycles along the axle, but also result in a synchronized motion of the two macrocycles when adding partial amount of stimuli. Moreover, the intermediate state with one shuttled macrocycle even exist predominantly in the solution during the titration of stimuli, which is theoretically unexpected for the [3]rotaxane with two non-interacting rings. This biomimetic strategy may provide a method for constructing highly complex molecular machines.