Solvophobically driven pi-stacking of phenylene ethynylene macrocycles andoligomers

Phenylene ethynylene macrocycles and oligomers with three different side-ch ain linking groups (ester, benzyl ether, and phenyl ether) were synthesized to investigate their tendency to undergo solvent induced pi -stacked organ ization. H-1 NMR, UV, and fluorescence spectroscopies were used to probe tw o types of pi -stacked supramolecular organizations: the intramolecular con formational ordering of the oligomers, and the intermolecular aggregation o f the macrocycles. One important conclusion is that solvent can play a very dramatic role in modulating the strength of the interactions that drive th e association of these ct-stacked structures. The other important conclusio n is that in a given solvent, the nature of the side chain linking group st rongly influences the pi -stacking propensities. It was found that macrocyc les and oligomers with the eater side chain linking group were prone to ado pt pi -stacked structures in a range of solvents, whereas the corresponding macrocycles with benzyl ether and phenyl ether side chain linking groups s howed only limited ability to pi -stack, even in the most polar solvent exa mined (DMSO). In the interest of manipulating the helix-coil folding transi tion of phenylene ethynylene oligomers, a heterosequence consisting of mono mers with ester and benzyl ether side chain linkages was synthesized. The f olding transition of the heterooligomer was intermediate to that observed f or the corresponding homooligomers, suggesting that the backbone sequence c an be used to tune the stability of conformations that are based on pi -sta cked organizations.