Molecular Design Strategy of π‐Conjugated Polymers for Efficient Visible‐Light‐Driven Photoelectrocatalytic O ₂ Reduction to H ₂ O ₂ Production

Toward sustainable hydrogen peroxide (H 2 O 2 ) production, photo(electro)catalytic oxygen (O 2 ) reduction/H 2 O 2 production has attracted increasing attention. Recently, we have found that a thin film of the π‐conjugated polymer, poly(1,4‐bis(2‐thienyl)benzene) ( PBTB ), exhibits exceptionally high (photo)electrocatalytic activity for O 2 reduction/H 2 O 2 production. To achieve higher photoelectrocatalytic activity and efficient visible‐light‐driven photoelectrocatalytic H 2 O 2 production, we investigated the molecular design related to the highest occupied molecular orbital (HOMO) energy level ( E HOMO ) of these polymers. We designed and synthesized poly(1,4‐bis(2‐thienyl)naphthalene) ( PBTN ), in which replacing the phenyl unit of PBTB with a naphthalene unit—a stronger electron‐withdrawing group—and increasing the polymer chain twist angle selectively deepened E HOMO relative to PBTB . The degree of E HOMO deepening quantitatively affected the onset potential of PBTN . Under visible‐light irradiation and 0 V vs. Ag/AgCl, the PBTN thin film achieved a high O 2 reduction/H 2 O 2 production rate (1.11 × 10 3 /g photoelectrocatalyst ), 1.47 times higher than that of PBTB , with excellent Coulombic efficiency (99%) and selectivity (99%). The onset potential of PBTN for visible‐light‐assisted O 2 reduction enabled a photocatalytic H 2 O 2 production setup. Upon visible‐light irradiation, this setup achieved a high photocatalytic O 2 reduction/H 2 O 2 production rate of 128 /g photocathode . These results clearly demonstrate the tunability of the photoelectrocatalytic activity of π‐conjugated polymers through E HOMO ‐related molecular design.