Mechanical Stirring Induces Heteroaggregate Formation and Self-assembly of Pseudoenantiomeric Oxymethylene Helicene Oligomers in Solution

A mixture of pseudoenantiomeric oxymethylene helicene (P)-pentamers and (M)-hexamers formed heteroaggregates and self-assembled as a result of mechanical stirring in solution at 25 degrees C. No aggregation occurred without stirring, and heteroaggregation was accelerated at high stirring rates. Repeatedly alternating between mechanical stirring and stopping respectively induced and then ceased the heteroaggregation. Sonication also promoted the heteroaggregation, which did not occur at high pressures or during centrifugation. Mechanisms based on friction due to mechanical stirring, which generated local and temporal high-temperature domains, are considered. It is shown that different methods of mechanical stimulation, namely, stirring and surface contact, induce enantiomeric heteroaggregate formation.