Post-polymerisation functionalisation of conjugated polymers: oligoether side-chain length governs exciton lifetime and dissociation for enhanced photocatalytic hydrogen evolution

Abstract

Conjugated polymers are promising organic photocatalysts for solar-driven hydrogen evolution. However, their performance is often limited by poor hydrophilicity, high exciton binding energy, and rapid charge recombination. In this work, we report a post-polymerisation functionalisation strategy. We graft oligoethylene glycol (OEG) side chains of varying lengths (N = 2, 4, 6, 8) onto the poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) backbone. The grafting is achieved via nucleophilic aromatic substitution. This yields a series of F8BT-OEGN polymers with identical conjugated main chains. Systematic characterisation confirms successful OEG incorporation. It also reveals that increasing OEG chain length progressively extends the fluorescence lifetime in the aggregated state (from 0.47 ns for F8BT-F to 2.59 ns for F8BT-OEG8). Longer chains also enhance the molecular dipole moment (up to 2.77 D for F8BT-OEG8). Moreover, the OEG side chains improve surface hydrophilicity and strengthen the built-in electric field. These effects are evidenced by Kelvin probe force microscopy and zeta potential measurements. We blend the polymers with the fullerene acceptor PCBM to form donor/acceptor heterojunction nanoparticles. The resulting F8BT-OEG8/PCBM photocatalyst achieves an outstanding hydrogen evolution rate of 50.4 mmol g⁻¹ h⁻¹. This rate significantly outperforms the unmodified counterpart. This work provides quantitative insights into the length-dependent effects of OEG side chains. It also demonstrates a robust post-modification approach to enhance photocatalytic performance.