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

Ladder polymers with backbones of uninterrupted ring-fused units have attracted academic and industrial attention for decades because of their extended pi-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 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 a seven-membered ring can be regioselectively and quantitatively generated postpolymerization. Model compounds with repeating units of polymers have also been synthesized, and X-ray crystallographic analysis reveals a nonplanar 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 pi electron oxepine ring driven by the excited-state aromatic stabilization energy, as indicated by the large Stokes shift.