Oxepine-Based π‑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 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 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 π
electron oxepine ring driven by the excited-state aromatic stabilization
energy, as indicated by the large Stokes shift.