Oxepine-Based π-Conjugated Ladder/Step-Ladder Polymers with Excited -State Aromaticity

Ladder polymers with backbones of uninterrupted ring-fused units have attracted academia and the industrial attention for decades because of their extended π-conjugation and intrinsic microporosity for possible potential applications in organic optoelectronics and membrane gas separations. We report herein the synthesis and characterization of a new series of oxepine-based ladder/step-ladder polymers, prepared by the acid-promoted intramolecular aromatic electrophilic cyclization reactions on alkyne-containing poly(arylene ether) precursors. In contrast to the more common annulations that produce five- and six-membered rings, we report that seven-membered ring can be regioselectively and quantitatively generated post-polymerization. Model compounds, with repeating units of polymers, have also been synthesized and X-ray crystallographic analysis reveals a non-planar contorted structure that is also present in the polymers. More interestingly, the oxepine-based ladder/step-ladder polymers appear to display a photoinduced planarization of the 8 π electron oxepine ring driven by the excited state aromatic stabilization energy and indicated by the large Stokes shift.