Molecular Engineering of Conjugated Acetylenic Polymers for Efficient Cocatalyst‐free Photoelectrochemical Water Reduction

Conjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene‐ to thiophene‐based. The polarized thiophene‐based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on‐surface Glaser polycondensation, as‐fabricated poly(2,5‐diethynylthieno3,2‐b]thiophene) on commercial Cu foam exhibits a record H₂‐evolution photocurrent density of 370 μA cm^?2 at 0.3 V vs. reversible hydrogen electrode among current cocatalyst‐free organic photocathodes (1–100 μA cm^?2). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high‐activity organic photoelectrodes.