Cooperative conformational transitions in phenylene ethynylene oligomers: Chain-length dependence

Fluorescence quenching has been used to study the cooperative conformationa l transition in a series of oligo(phenylene ethynylene)s having tri(ethylen e glycol) monomethyl ether side chains. In nonpolar solvents such as chloro form, the intensity of fluorescence emission from the backbone chromophore increases smoothly as the chain lengthens from the dimer through the octade camer. In polar solvents such as acetonitrile, on the other hand, chains ha ving more than eight units exhibit fluorescence quenching concomitant with the growth of an intramolecular excimer-like band. This observation is cons istent with pi-stacking of aromatic rings for chains that are long enough t o fold back on themselves, driven by solvophobic interactions. Titration ex periments in which the-solvent composition was gradually changed from pure acetonitrile to pure chloroform showed sigmoidal curves characteristic of a cooperative transition. These data were analyzed using a two-state approxi mation and a model in which the free energy difference between conformation al states depends linearly on solvent composition. The stability of the ord ered state was found to increase linearly with chain length, suggestive of a;regularly repeating conformation such as a helix. Therefore the data were fit to a two-state helix-coil equilibrium model where good agreement was o bserved. The parameters obtained from this analysis revealed a highly coope rative transition driven to fold by strongly interacting monomer pairs.