Search

We report on the use of atomic force microscopy (AFM) to identify and characterize an intermediate state in macrocycle shuttling in a hydrogen bonded amide-based molecular shuttle. The [2]rotaxane consists of a benzylic amide macrocycle mechanically locked onto a thread that bears both fumaramide and succinic amide-ester sites, each of which can bind to the macrocycle through up to four intercomponent hydrogen bonds. Using AFM-based single-molecule force spectroscopy, we mechanically triggered the translocation of the ring between the two principal binding sites ("stations") on the axle. Equilibrium fluctuations reveal another interacting site involving the two oxygen atoms in the middle of the thread. We characterized the ring occupancy distribution over time, which confirms the intermediate in both shuttling directions. The study provides evidence of weak hydrogen bonds that are difficult to detect using other methods and shows how the composition of the thread can significantly influence the shuttling dynamics by slowing down the ring motion between the principal binding sites. More generally, the study illustrates the utility that single-molecule experiments, such as force spectroscopy, can offer for elucidating the structure and dynamics of synthetic molecular machines.

Donor-acceptor (DA) π-interactions are weak attractive forces that are exploited widely in molecular and supramolecular chemistry. They have been characterized extensively by ensemble techniques, providing values for their energies that are useful for the design of soft materials. For implementation of motions or operations based on these DA π-interactions in wholly synthetic molecular machines, the mechanical strength and force associated with their out-of-equilibrium performance are the key parameters, in addition to their energies obtained at thermodynamic equilibrium. In this context, we have used single-molecule force spectroscopy as a nonequilibrium technique to determine the mechanical strength of individual DA π-interactions in solution. We designed and synthesized a molecular tweezer that is able to encapsulate π-donors and also demonstrated a precise opening extension. The mechanical breaking of the noncovalent interactions between viologen units-π-acceptors commonly employed in mechanically interlocked molecules-and several π-donors afforded a characteristic force-distance signature, revealing the opening of individual viologen tweezers with an unambiguous extension. Single-tweezer host-exchange experiments performed

Artificial molecular pumps (AMPs), inspired by the active cellular transport exhibited in biological systems, enable cargoes to undergo unidirectional motion, courtesy of molecular ratchet mechanisms in the presence of energy sources. Significant progress has been achieved, using alternatively radical interactions and Coulombic repulsive forces to create working AMPs. In an attempt to widen the range of these AMPs, we have explored the effect of molecular pumping on the photophysical properties of a collecting chain on a dumbbell incorporating a centrally located pyrene fluorophore and two terminal pumping cassettes. The AMP discussed here sequesters two tetracationic cyclophanes from the solution, generating a [3]rotaxane in which the fluorescence of the dumbbell is quenched. The research reported in this Article demonstrates that the use of pumping cassettes allows us to generate the [3]rotaxane in which the photophysical properties of fluorophores can be modified in a manner that cannot be achieved with a mixture of the dumbbell and ring components of the rotaxane on account of their weak binding in solution.