Why do sulfone-containing polymer photocatalysts work so well for sacrificial hydrogen evolution from water?

In recent years, many of the highest performing polymer photocatalysts for hydrogen evolution from water have contained dibenzo[b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We use a new set of processable materials, in which the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Using transient absorption spectroscopy and electrochemical measurements, and combined with molecular dynamics and density functional theory simulations, we find that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased, and we demonstrate here that TEA oxidation is only thermodynamically favourable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest performing, sulfone-rich material, hydrogen evolution appears to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.