Topological Dynamics in Supramolecular Rotors

Artificial molecular switches, rotors,
and machines are set to
establish design rules and applications beyond their biological counterparts.
Herein we exemplify the role of noncovalent interactions and transient
rearrangements in the complex behavior of supramolecular rotors caged
in a 2D metal–organic coordination network. Combined scanning
tunneling microscopy experiments and molecular dynamics modeling of
a supramolecular rotor with respective rotation rates matching with
0.2 kcal mol–1 (9 meV) precision, identify key steps
in collective rotation events and reconfigurations. We notably reveal
that stereoisomerization of the chiral trimeric units entails topological
isomerization whereas rotation occurs in a topology conserving, two-step
asynchronous process. In supramolecular constructs, distinct displacements
of subunits occur inducing a markedly lower rotation barrier as compared
to synchronous mechanisms of rigid rotors. Moreover, the chemical
environment can be instructed to control the system dynamics. Our
observations allow for a definition of mechanical cooperativity based
on a significant reduction of free energy barriers in supramolecules
compared to rigid molecules.