A Folded, Secondary Structure in Step-Growth Oligomers from Covalently Linked, Crowded Aromatics

This study delineates general methods to create a new class of folded oligomers by covalently attaching overcrowded aromatics to each other. Crucial to observing the secondary structure in these oligomers was the employment of C-shaped linkers. These linkers preorganize the strands to form intramolecular hydrogen bonds. In solution, one- and two-dimensional 1H NMR data show well-defined columnar conformations. The side chains in these oligomers are critical for the secondary structure to emerge in solution. Using tris(dodecyloxy)phenethyl side chains in combination with tert-butyl side chains in the terminal subunit provides a soluble trimer and prevents intermolecular association above millimolar concentrations. This new folding motif, formed through a synergy between hydrogen bonds and π-stacking, is so robust that even dimers have secondary structure in solution.