Cooperative self-assembly of oligo(m-phenyleneethynylenes) into supramolecular coordination polymers

Here we report the supramolecular coordination polymerization of four bis-functionalized phenyleneethynylene oligomers. Each oligomer was terminated with pyridyl end groups; three of them had meta backbone linkages and varied in the number of phenyleneethynylene units from four to eight. The fourth oligomer served as a geometric control with para connectivity and three phenyleneethynylene units. Upon the addition of trans-dichlorobis(acetonitrile) palladium to a solution of oligomer, supramolecular assembly by metal coordination was observed by UV spectroscopy, NMR, and isothermal titration calorimetry. For the tetramer and octamer, the experimental evidence is consistent with a cooperative polymerization mechanism involving pyridine-palladium coordination and pi-stacking leading to a helical supramolecular polymer. For the hexamer, macrocyclization followed by columnar aggregation was indicated by the data. In contrast, the para-linked control oligomer exhibited noncooperative, isodesmic step-growth polymerization. This work thus demonstrates how the supramolecular polymerization mechanism is sensitive to length of the starting oligomer and geometry of the monomer units.