Folding and aggregation of backbone-rigidified oligo(m-phenylene ethynylenes) in polar and nonpolar media

The folding and chiral aggregation of backbone-rigidified oligo(m-phenylene ethynylenes) were examined using NMR, UV, circular dichromism (CD) and dynamic light scattering (DLS) in methanol and chloroform. Evidence from 2D NMR suggests that backbone-rigidifying intramolecular H-bonds persist in chloroform and methanol, suggesting that these molecules are stably folded in both nonpolar and polar solvents. Results from CD studies further confirmed that longer oligomers (pentamer and hexamer) adopted chiral (helical) conformations. DLS indicated that, in chloroform, the oligomers are fully solvated and exhibited no aggregation; in methanol, these molecules show a size-dependent aggregation, in which a pentamer and a hexamer formed large assemblies while a smaller tetramer showed no detectable aggregation.