While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic photocatalysts are currently gaining substantial momentum - particularly due to their much higher synthetic flexibility. For instance, their optical band gap can be tuned continuously throughout large parts of the solar spectrum by copolymerising suitable monomers in defined ratios.¹ This tunability has sparked intense research interest in organic photocatalysts,^2,3 however, the fundamental understanding of photoinduced processes in these systems has stayed behind the rapid development of new materials. Some parallels can be drawn to organic photovoltaics where comparable materials are used, but especially the aqueous environment makes polymer photocatalysts distinct from other applications. To understand what dictates their performance and how structurally similar polymers can exhibit very different degrees of hydrogen evolution activity,⁴ photophysical processes in these materials require further investigation.

The combined study presented here is the first in-depth investigation of hydrogen evolution activity of linear conjugated polymers and combines materials development with spectroscopic characterisation and computational modelling. We investigate a series of polymers with strikingly different hydrogen evolution activity, including some of the highest performing photocatalysts reported to date in this class of materials. A comparison to structurally related polymers with significantly lower activity allows us to identify the key determinants of hydrogen evolution activity in this series. To this end, we use transient absorption spectroscopy to monitor photogenerated reaction intermediates on time sales of femtoseconds to seconds after light absorption and correlate the type and yield of observed intermediates with the hydrogen evolution activity of the respective polymer. Computational simulations and calculations build on this transient data and extend the observations to the role of the solvent environment in the photoinduced reaction sequence. The presented results can provide design strategies for new materials and thus have implications for the development of more efficient organic photocatalysts.

References

1. Sprick, R. S. et al. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 137, 3265–3270 (2015).

2. Zhang, G., Lan, Z.-A. & Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chemie Int. Ed. 55, 15712–15727 (2016).

3. Vyas, V. S., Lau, V. W. & Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 28, 5191–5204 (2016).

4. Sprick, R. S. et al. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chemie Int. Ed. 55, 1792–1796 (2016).