While the field of sunlight-driven fuel generation has traditionally been dominated by inorganic materials, organic semiconductors are currently gaining substantial momentum for application as photocatalysts - 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 copolymerizing selected monomers in defined ratios. This tunability has sparked intense research interest in organic photocatalysts, however, the fundamental understanding of photoinduced processes in these systems and the characterisation of their catalytically active sites have stayed behind the rapid development of new materials.
In this presentation, I will demonstrate how transient and operando optical spectroscopic techniques can be used to track the evolution of photogenerated reaction intermediates in polymer photocatalysts on timescales of femtoseconds to seconds after light absorption. To this end, short laser pulses are used to study these photocatalysts under transient conditions whereas long LED pulses are employed to establish operando catalytic conditions, and photogenerated reaction intermediates are then probed optically. Firstly, these techniques reveal insights into the yield of photogenerated charges, which enables an understanding of differences in hydrogen evolution activity between different materials. Secondly, these techniques allow to monitor the transfer of photogenerated electrons to catalytically active sites as well as their accumulation under operando photocatalytic conditions, where differences in electron transfer time translate into different kinetic bottlenecks of the hydrogen evolution reaction for different polymers. To illustrate these points, I will draw direct comparisons between nanoparticle photocatalysts made from the polymers F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer, P10, which is one of the most performant polymer photocatalysts reported to date.
 Wang, Y.; Vogel, A.; Sachs, M.; Sprick, R. S.; Wilbraham, L.; Moniz, S. J. A.; Godin, R.; Zwijnenburg, M. A.; Durrant, J. R.; Cooper, A. I.; et al. Current Understanding and Challenges of Solar-Driven Hydrogen Generation Using Polymeric Photocatalysts. Nat. Energy 2019, 4 (9), 746–760. https://doi.org/10.1038/s41560-019-0456-5.
 Sachs, M.; Sprick, R. S.; Pearce, D.; Hillman, S. A. J.; Monti, A.; Guilbert, A. A. Y.; Brownbill, N. J.; Dimitrov, S.; Shi, X.; Blanc, F.; et al. Understanding Structure-Activity Relationships in Linear Polymer Photocatalysts for Hydrogen Evolution. Nat. Commun. 2018, 9 (1), 4968. https://doi.org/10.1038/s41467-018-07420-6.
 Sachs, M.; Cha, H.; Kosco, J.; Aitchison, C. M.; Francàs, L.; Corby, S.; Chiang, C.-L.; Wilson, A. A.; Godin, R.; Fahey-Williams, A.; Cooper, A. I.; Sprick, R. S.; McCulloch, I.; Durrant, J. R. Tracking Charge Transfer to Residual Metal Clusters in Conjugated Polymers for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2020, 142 (34), 14574–14587. https://doi.org/10.1021/jacs.0c06104.