Mechanoresponsive functional polymers are crucial in the actual developments of materials resistant to external mechanical tension. Such polymers can be based on covalently incorporated mechanophores or the assembly of non-covalent binding motifs. Noncovalent interactions support the conformational changes of the polymers or controlled intramolecular motions. The mechanical strength and force of such weak bonding interactions had never been quantified individually. Quantifying the force that synthetic molecules can generate is however essential to design functional materials or artificial molecular machines able to perform specific tasks, e.g., catalysis or transport. Recently, using AFM-based single-molecule force spectroscopy, we have demonstrated that synthetic foldamers, based on a combination of donor-acceptor π-interactions and H-bonds, could generate considerable forces.1-2 Here we decipher the influence of the donor-acceptor π-interactions responsible for this performance and quantify directly the force required to break these individual π interactions.3 We designed and synthesized a π acceptor viologen tweezer capable of hosting various π-donors. We employed AFM to exert a mechanical force on the tweezer to probe the mechanical strength of the interaction. Force experiments revealed a clear signature for the mechanical opening of a single tweezer, thus confirming the study at the single-interaction level. The sensitivity of our approach was assessed with different π-donors and confirmed by in situ exchange experiments in solution. This strategy allows us to probe, for the first time, the mechanical strength of donor-acceptor π-interactions at the single bond level. It opens up the possibility to describe chemical interactions in terms of force, or work performance, instead of thermodynamic equilibrium energetic values such as association constants.

Quantifying the mechanical strength of individual π-interactions by AFM

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ACS Fall 2022

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