Organic Polymer and Molecular Nanoparticles for Photocatalytic Hydrogen Production : Insights into Excited-State Dynamics

The growing need for sustainable energy solutions has increased research efforts in artificial photosynthesis, particularly photocatalytic systems that convert solar energy into chemical fuels. Among these, organic nanoparticle (NP) photocatalysts have shown to be promising candidates for hydrogen evolution due to their earth-abundant composition, structural tunability, and ability to absorb visible light. However, their performance can only be improved through a more detailed understanding of the mechanisms involved. This thesis investigates how exciton dynamics, excited-state lifetimes, and molecular architecture influence the photocatalytic performance of organic NP systems. Using steady-state and time-resolved spectroscopic techniques across femtosecond to second timescales, this study provides new insights into the fundamental photophysical processes governing hydrogen evolution. Paper I demonstrates that the exciton diffusion length in the commonly used polymer NP photocatalyst PFBT significantly exceeds previous estimates, indicating that excitons can reach the NP-water interface before recombination with high probability. This finding is important because the accessibility of excitons at the interface determines whether charges can separate and participate in photocatalytic reactions. Paper II shows that molecular design strategies, such as incorporating strong electron-withdrawing units, adopting star-shaped geometries, and having favourable NP morphologies, synergistically enhance hydrogen evolution performance. These improvements are additionally linked to the formation of long-lived triplet charge-transfer states. Papers III and IV explore NP systems based on non-fullerene acceptors, which are widely used in organic photovoltaics, but underexplored in photocatalysis. Paper III reveals that Y5 NPs exhibit higher hydrogen evolution rates than Y6 NPs due to thermally activated delayed fluorescence and more favourable reduction potentials, contrasting with trends observed in photovoltaics. Paper IV introduces a newly designed A–D–A molecule, IT-PMI, which promotes intramolecular charge transfer and intermolecular interactions. This design enables charge hopping between chromophores, leading to improved photocatalytic activity and enhanced photostability compared to Y-series NPs.