Solvophobically driven π-stacking of phenylene ethynylene macrocycles and oligomers

Phenylene ethynylene macrocycles and oligomers with three different side-chain linking groups (ester, benzyl ether, and phenyl ether) were synthesized to investigate their tendency to undergo solvent induced π-stacked organization. ¹H NMR, UV, and fluorescence spectroscopies were used to probe two types of π-stacked supramolecular organizations: the intramolecular conformational ordering of the oligomers, and the intermolecular aggregation of the macrocycles. One important conclusion is that solvent can play a very dramatic role in modulating the strength of the interactions that drive the association of these π-stacked structures. The other important conclusion is that in a given solvent, the nature of the side chain linking group strongly influences the π-stacking propensities. It was found that macrocycles and oligomers with the ester side chain linking group were prone to adopt π-stacked structures in a range of solvents, whereas the corresponding macrocycles with benzyl ether and phenyl ether side chain linking groups showed only limited ability to π-stack, even in the most polar solvent examined (DMSO). In the interest of manipulating the helix-coil folding transition of phenylene ethynylene oligomers, a heterosequence consisting of monomers with ester and benzyl ether side chain linkages was synthesized. The folding transition of the heterooligomer was intermediate to that observed for the corresponding homooligomers, suggesting that the backbone sequence can be used to tune the stability of conformations that are based on π-stacked organizations.