Poly(pentafulvalene)s: One-Dimensional Conjugated Hydrocarbons toward Stable and Narrow-Bandgap Polyenes

Polyacetylene is the archetypal narrow-bandgap conjugated polymer, yet its high-lying HOMO renders it air-sensitive, and aromatic alternatives sacrifice the small bandgap inherent to a polyene backbone. Here we show that embedding the polyene in a dibenzopentafulvalene (DBF) framework affords a stable, narrow-bandgap polyene analog. Poly(dibenzopentafulvalene) (polyDBF) was synthesized by Kosugi–Migita–Stille polycondensation; its thin film absorbed up to 995 nm (optical bandgap 1.25 eV) and, unlike polyacetylene, persisted for days in air. Because intermolecular interactions in the solid state and chain-length dispersity preclude isolating the intrinsic electronic structure of each polymer, we synthesized soluble, monodisperse oligomers [1]DBF–[5]DBF up to the pentamer. Extrapolation of their longest-wavelength absorptions yielded an intrinsic optical bandgap of 1.64 eV for the ideal polymer, which is substantially narrower than that of polyene (2.03 eV) and even than PEDOT (1.92 eV), a benchmark narrow-bandgap homopolymer, as obtained by the same analysis. Quantum-chemical calculations trace this narrowing to the combined effects of cross-conjugation and five-membered-ring cyclization, which selectively lower the LUMO. Consistent with our earlier oligomer series up to [3]DBF, the longer oligomers further sustain reversible multi-electron reduction, reaching an eight-electron reduction in [4]DBF. These results establish DBF as a building block that narrows the polyene bandgap while conferring ambient stability, and provide a design principle for stable narrow-bandgap hydrocarbon homopolymers grounded in their intrinsic electronic structure.