Three series of conjugated microporous polymers (CMPs) were studied as photocatalysts for hydrogen production from water using a sacrificial hole scavenger. In all cases, dibenzo[b,d]thiophene sulfone polymers outperformed their fluorene analogues. A porous network, S-CMP3, showed the highest hydrogen evolution rates of 6076 μmol h −1 g −1 (λ > 295 nm) and 3106 μmol h −1 g −1 (λ > 420 nm), with an external quantum efficiency of 13.2% at 420 nm. S-CMP3 outperforms its linear structural analogue, P35, whereas in other cases, nonporous linear polymers are superior to equivalent porous networks. This suggests that microporosity might be beneficial for sacrificial photocatalytic hydrogen evolution, if suitable linkers are used that do not limit charge transport and the material can be wetted by water as studied here by water sorption and quasi-elastic neutron scattering.

Structure–property–activity relationships in solution processable polymer photocatalysts for hydrogen production from water were probed by varying the chemical structure of both the polymer side-chains and the polymer backbone.

exhibit photocatalytic hydrogen evolution in 2009, [12] and many advances have been made since then. [13,14] After the potential of g-C 3 N 4 was first observed, while focusing on the hydrogen evolution half-reaction, interest has begun to shift to achieving overall water splitting using these materials. [15,16] However, the exact structure of most g-C 3 N 4 materials is unknown and the synthesis usually involves high temperature processing, which offers limited scope for fine-tuning structure and properties. Also, while g-C 3 N 4 can be produced from inexpensive starting materials, the synthetic yield of the material is typically low. [15,17] Of special relevance here, graphitic carbon nitrides are insoluble solids: as for many inorganic catalysts, this can present challenges in terms of processing.Rather few organic photocatalysts have been studied for hydrogen evolution other than g-C 3 N 4 . Recently, nitrogencontaining poly(azomethine) networks and covalent triazinebased frameworks (CTFs) were shown to have photocatalytic activity with the addition of platinum cocatalysts. [18,19] We have shown that a series of conjugated microporous polymers (CMPs) could facilitate hydrogen evolution from water in the presence of a sacrificial electron donor, without any additional heavy metal cocatalyst. [20,21] Other CMPs have since been studied for photocatalysis [22,23] and recent studies have demonstrated that linear conjugated polymers can have high photocatalytic activities. [24,25] However, as with g-C 3 N 4 , none of these organic materials are soluble in common organic solvents. This insolu bility makes it more challenging to process these materials into functional composites. Moreover, photocatalysts are typically kept in suspension by stirring to prevent sedimentation, which results in loss of photocatalytic activity. [26] The loss of activity of insoluble catalysts can be prevented with the use of support substrates, [27] however, using solution processability allows the use of simpler supports and easier development of photoelectrodes.Soluble oligo(phenylene)s have been previously reported as photocatalysts, however, they displayed low activity, were only active under UV light, required a Ru cocatalyst and were only poorly soluble in organic solvents limiting processability. [28] More recently soluble metal-chelating polymers have been prepared although the photocatalytic activity of these polymers also appear to be very low with apparent quantum yields (AQY) below 3 × 10 −4 %. [29] The solubility of some alkylated conjugated polymers has also facilitated the preparation of polymer nanoparticles (PDots). [30,31] The preparation of these PDots enabled significant enhancements in rate over the pristine polymer although scalability and long-term stability of this approach has yet to be shown. Direct photocatalytic water splitting is an attractive strategy for clean energy production, but multicomponent nanostructured systems that mimic natural photosynthesis can be difficult to fabricate because of the insolubil...

The photocatalytic performance of fluorene-type polymer photocatalysts for hydrogen production from water in the presence of a sacrificial hole scavenger is significantly improved by the incorporation of heteroatoms into the bridge-head through a combination of factors.

Many of the highest-performing polymer photocatalysts for sacrificial 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 study films, dispersions, and solutions of a new set of solution-processable materials, where 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. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show 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. We show that TEA oxidation is only thermodynamically favorable 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 seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.