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

Abstract

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/stepladder 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 sevenmembered 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.

Ambipolar conjugated ladder polymers by room-temperature Knoevenagel polymerization

“…23 Since the report of the first soluble cLP ( i.e. , ladder conjugated poly( p -phenylene), LPPP) in 1991, 27 there has been an ever-growing interest in this stepwise technique, thereby providing a wide scope of monomeric building blocks and applicable intramolecular ring-closure reactions, including Scholl coupling, 14 electrophilic cyclization, 27–31 photocyclization, 25,32,33 Schiff base condensation, 34,35 and olefin ring-closing metathesis. 36,37 The other strategy for the preparation of cLPs is a single-step ladderization route that involves a simultaneous formation of a two-stranded bond between comonomers.…”