Mechanochemical Cycloreversion of Cyclobutane Observed at the Single Molecule Level

Area of Science:

• Chemistry
• Materials Science
• Nanotechnology

Background:

• Mechanochemical cycloreversion of cyclobutane has been observed using ultrasound, but the precise forces required remain unclear.
• Assigning specific bond ruptures in atomic force microscopy (AFM) experiments is challenging.

Purpose of the Study:

• To determine the forces necessary to induce mechanochemical cycloreversion of cyclobutane.
• To develop a method for unambiguously assigning bond rupture in AFM experiments.

Main Methods:

• Synthesis of tailored macrocycles containing a cyclobutane mechanophore bypassed by an ethylene glycol chain.
• Covalent anchoring of macrocycles to a glass substrate and AFM cantilever using polyethylene glycol linkers.
• Mechanical stretching of macrocycles and identification of cycloreversion via polymer length increase.

Main Results:

• Cycloreversion was identified by a defined increase in polymer length upon mechanical stretching.
• The measured length change correlated with calculations using the external force explicitly included (EFEI) method.
• Using two different ethylene glycol chain lengths provided unambiguous assignment of cycloreversion.
• Mechanochemical cycloreversion of cyclobutane was observed at forces exceeding 1.7 nN.

Conclusions:

• The study successfully determined the force threshold for mechanochemical cycloreversion of cyclobutane.
• Tailored macrocycles with safety lines offer a reliable method for studying mechanochemistry in AFM.
• This approach provides a clear assignment of bond rupture events in single-molecule force spectroscopy.