Site-specific fluorination on donor-acceptor polymers enhances intramolecular charge transfer for photocatalytic hydrogen evolution

Abstract

The development of polymer photocatalysts remains a central challenge in achieving efficient solar-to-fuel conversion. A key limitation lies in the use of weak donor units, which hampers the optimization of intramolecular donor-acceptor interactions and reduces charge-transfer efficiency. Here, we report a site-specific fluorination strategy that addresses this bottleneck by introducing fluorine atoms at defined positions along the polymer backbone via a unique two-step polymerization protocol. Two representative random copolymers were synthesized: the acceptor-fluorinated polymer (PBF8BT-AF) and the donor-fluorinated polymer (PBF8BT-DF). Strikingly, PBF8BT-DF exhibited superior photocatalytic activity, while PBF8BT-AF showed inferior performance due to exciton loss through a non-productive decay pathway. A comprehensive spectroscopic analysis, supported by density functional theory (DFT) calculations, reveals that donor-site fluorination promotes exciton delocalization across the entire repeating unit, thereby facilitating efficient intramolecular charge transfer. In contrast, acceptor fluorination energetically localizes the excited state, impeding exciton migration and suppressing charge utilization. These findings underscore the critical role of site-specific exciton stabilization in determining photocatalytic efficiency and establish domain-targeted molecular engineering as a powerful design principle for next-generation polymer photocatalysts.