Exploring the "Goldilocks Zone" of Semiconducting Polymer Photocatalysts via Donor-Acceptor Interactions

Water splitting using polymer photocatalysts is a key technology to a truly sustainable hydrogen-based energy economy. Synthetic chemists have intuitively tried to enhance photocatalytic activity by tuning the length of π-conjugated domains of their semiconducting polymers, but the increasing flexibility and hydrophobicity of ever-larger organic building blocks leads to adverse effects such as structural collapse and inaccessible catalytic sites. To reach the ideal optical bandgap of ~2.3 eV, we synthesised a library of eight sulphur and nitrogen containing porous polymers (SNPs) with similar geometries but with optical bandgaps ranging from 2.07 to 2.60 eV using Stille coupling. These polymers combine π-conjugated electron-withdrawing triazine- (C₃N₃) and electron donating, sulphur-containing moieties as covalently-bonded donor-acceptor frameworks with permanent porosity. The remarkable optical properties of SNPs enable fluorescence on-off sensing of volatile organic compounds and illustrate intrinsic charge-transfer effects. Moreover, obtained polymers effectively evolve H₂ gas from water under visible light irradiation with hydrogen evolution rates up to 3158 µmol h^-1 g^-1 and high apparent quantum efficiency which is the highest value obtained for microporous organic polymers to-date. The design principles demonstrated here are transferable to a new field of high-performance polymer photocatalysts based on efficient donor-acceptor dyads.