Molecular Design Strategy of π-Conjugated Polymers for Efficient Visible-Light-Driven Photoelectrocatalytic O₂ Reduction to H₂O₂ Production.

Toward sustainable hydrogen peroxide (H₂O₂) production, photo(electro)catalytic oxygen (O₂) reduction/H₂O₂ 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 O₂ reduction/H₂O₂ production. To achieve higher photoelectrocatalytic activity and efficient visible-light-driven photoelectrocatalytic H₂O₂ production, we investigated the molecular design related to the highest occupied molecular orbital (HOMO) energy level (EHOMO) 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 EHOMO relative to PBTB. The degree of EHOMO 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 O₂ reduction/H₂O₂ production rate (1.11 × 10³ mmol H 2 O 2 (mmol)_H₂ O₂ /gphotoelectrocatalyst), 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 O₂ reduction enabled a photocatalytic H₂O₂ production setup. Upon visible-light irradiation, this setup achieved a high photocatalytic O₂ reduction/H₂O₂ production rate of 128 mmol H 2 O 2 (mmol)_H₂ O₂ /gphotocathode. These results clearly demonstrate the tunability of the photoelectrocatalytic activity of π-conjugated polymers through EHOMO-related molecular design.