Regioisomeric control of planarity enhances exciton dissociation in conjugated polymers for high-efficiency photocatalytic H2 evolution

Abstract

Organic conjugated polymers (CPs) are promising photocatalysts for solar-to-hydrogen conversion, yet their efficiency is often limited by large exciton binding energies and poor charge separation. In this study, we report a molecular isomerization strategy to control polymer planarity and thereby enhance exciton dissociation and photocatalytic activity. Two isomeric CPs, Pyrene-Thiophene-Benzothiazole (Py-T-BT) and Pyrene-Thiophene-iso-Benzothiazole (Py-T-isoBT), were designed and synthesized, employing pyrene as the donor, thiophene as the π-bridge, and either benzothiadiazole (BT) or its structural isomer (isoBT) as the acceptor. The introduction of the isoBT unit induced steric hindrance, resulting in Py-T-isoBT exhibiting significantly reduced molecular planarity (dihedral angles of 30.3° and 9.2°) compared to the highly planar Py-T-BT (dihedral angle of 4.2°). Theoretical calculations and experimental characterization confirmed that the more planar Py-T-BT possessed a narrower band gap, a larger transition dipole moment, and consequently a significantly lower exciton binding energy. Time-resolved spectroscopy revealed that Py-T-BT enabled ultrafast formation of charge transfer excitons (τ_CT = 0.60 ps), in contrast to Py-T-isoBT (τ_CT = 3.95 ps). As a result, Py-T-BT demonstrated a photocatalytic hydrogen evolution rate of 7.95 mmol g⁻¹ h⁻¹ with a 3 wt% Pt cocatalyst under visible light irradiation (λ ≥ 420 nm), approximately 20 times higher than that of Py-T-isoBT. This work highlights the pivotal role of molecular planarity in modulating exciton dynamics and presents a generalizable strategy for designing high-efficiency polymer photocatalysts through isomeric engineering.